If you work in cramped surroundings, or you’re looking to add a second computer to your set-up, a small-format computer might be your saving grace.

Tired of banging your knees against the truck-sized tower lurking under your desk? Thinking about adding a second computer at your workstation, but put off by the bulk and noise of two systems? Contrary to conventional thinking, a PC doesn't have to be big or noisy. Modestly priced, moderately powerful, and often stylishly designed, these machines can be a great choice wherever space is at a premium.

The small PCs we saw fall into three categories: mini-boxes, thin-profile units, and all-in-ones. The last two categories have been around for years, mainly in the corporate world, but mini-boxes are relatively new – a format made viable as computer components got smaller and more powerful.

Apple's Mac Mini – a capable computer stuffed into a square case smaller than a lunch box – has brought a lot of attention onto the market for small PCs.

All of the leading names in computing – Dell, Gateway, HP, and IBM – make thin-profile models that sit flat on a desk or, when put on their edges, look like exceptionally narrow towers. They differ from mini-box systems only in the layout of their components. As thin towers, any of them can look fairly elegant alongside a slim display. If even that much of a footprint on your desk is too much, consider an all-in-one configuration: basically a computer bolted to the back of an LCD monitor for maximum space efficiency. Add a wireless keyboard and mouse to one of these systems, and you can reduce your cable clutter to almost nil.

A small PC shares the same shortcomings as notebook PCs: limited expandability and lower performance than full-size desktops offer. Computer users who love to tinker with and upgrade their systems will find these small units unappealing, as will power users. In our speed tests, the average WorldBench 5 score for the small systems we appraised was 83 – a far cry from the 110 and higher marks we've recorded for the fastest power models in our Top 15 Desktop PCs chart.

Small systems will suit users who don't upgrade much – just memory, the hard drive, or (if the unit has a PCI, AGP, or PCI Express slot) graphics, say. Small units will also suit people who want to buy a machine, set it in place, and never open the case (still the majority of PC users).

With a small PC, your best expansion option is via USB ports; most such systems have at least four ports for external hard drives and other peripherals. But that strategy can be self-defeating – as additional devices blossom alongside your system, the overall footprint grows, and not particularly neatly. You can end up with so much clutter that you might eventually decide to go back to a spacious tower that fits all the extras inside.

Hot stuff

The biggest challenge for small PCs isn't component size, but heat. Processors, hard disks, power supplies, and other PC components generate heat, and system makers have learned that just cramming standard components into a smaller case tends to create too hot a product. PC makers have solved this problem in a variety of ways, some obvious and others quite subtle. For instance, one of our all-in-one units, from UK maker Pelham Sloane, uses a Pentium M notebook CPU for cooler operations. Another solution – getting rid of the on-board power supply and using an AC ‘brick’ adaptor instead – removes a major source of heat and noise from the case but adds to the annoying litter of bricks already strewn under your desk.

A more complex technique involves carefully arranging internal components, segmenting the interior of the case into discrete airflow zones, and strategically situating fans and vents. The Shuttle XPC i8600b we tested provides the best example of how these techniques can reduce heat and noise in a small system. The machine’s dual 3.5-inch internal drive bays occupy a cage at the very top of the system. You can access the cage – which relies on its own airflow source and uses rubber isolators to shield the drives from vibrations – without tools. The power supply sits below the cage and vents through the back panel of the case, while the motherboard and processor cool off via a set of vents recessed into the front edges of the case. As a result, the Shuttle can rely on quiet, software-controlled, low-rotation fans for air circulation.

The ten small systems looked at here include two mini-boxes: Apple's Mac Mini and Shuttle's XPC i8600b. The thin-profile PCs – units that can sit flat or stand on edge – are Dell's OptiPlex SX280, Gateway's E-4300 4-Bay SB (Gateway does no operate in the UK), HP Compaq's Dc7100 Ultra-Slim Desktop, and IBM's ThinkCentre A50. Gateway's Profile 5.5C, MPC's ClientPro 414 All-in-One, Pelham Sloane's PS1500M, and Sony's VAIO VGC-V520G TV-PC make up our all-in-one group. The Dell and HP thin-profile machines can mount behind their monitors to form all-in-one PCs.

Mighty mini-boxes

Shuttle pioneered the toaster-size (or shoe box-size) PC design, and the XPC i8600b shows the company's mastery of the art of packing a lot into a small space. Though the case is only 8 inches high and just under 15 inches long, it contains a sample of pretty much everything a full-size PC offers, including a PCI Express slot, two hard-drive bays, and a plethora of expansion ports. It's also the only system in this roundup with support for surround-sound audio.

In spite of its small size, the Shuttle can pack enough horsepower to attract gamers: The box is easy to carry to LAN parties, and Shuttle's XP17 LCD, which came with our review unit, has a convenient carrying handle and a padded case.

Cute isn't a word we use often to describe personal computers, but it certainly applies to Apple's Mac Mini, the latest and least of the mini-boxes. Smaller computers exist, but they target niche uses. Even in the context of Apple's previous design achievements, the Mini is visually striking in its lack of features. A silver-and-white box measuring just 6.5 inches square (about the size of a CD case) and barely 2 inches high, the Mini includes only an optical disc slot on what passes for a front panel. The power button is on the back (a design flaw, we believe), along with video, USB, and FireWire ports.

Because our benchmark test suite focuses on Windows-based PCs, we cannot measure the Mini's performance against that of the other small models we reviewed. But sidestepping the issue of whether OS X or Windows XP is the better operating system, we enjoyed using the Mini: It was easy to set up, came with a nice software starter bundle that included video editing tools, and ran almost silently – the cooling fan produced just the barest whisper. It's easy to picture a Mini going places a typical desktop PC won't: into the kitchen, for example, or the ad hoc home office that pops up on the dining-room table every evening.

Though marketed to compete against cheap PCs, the £339 price for the most basic configuration doesn't include a keyboard, mouse, or monitor, but you can use any USB mouse and keyboard, and any DVI or VGA monitor. You'll also need a USB hub to expand the Mini, as it has only two USB 2.0 ports and the keyboard and mouse use one of those.

Your choice: flat or thin

The thin-profile systems we looked at from Dell, Gateway, HP, and IBM are marketed primarily as low-cost business machines that corporations can buy and deploy in large numbers.

Thin-profile models are proliferating as system makers embrace the slim-PC concept. Some vendors now offer several sizes of small, thin systems. When tipped on edge, the systems we reviewed ranged from 10 to 13 inches tall, from 10 to 17 inches deep, and from 3 to 4 inches wide.

The Dell OptiPlex SX280 packs a lot into its small case, fitting a 3.4-GHz Pentium 4 and 512MB of RAM into a box that's just over 10 inches long and 3.5 inches wide. But this leaves no room inside for expansion. The system lacks open PCI slots. The compact case can mount behind Dell's LCD monitor to transform the components into an all-in-one system with an even smaller footprint on your desk. We were impressed by the OptiPlex's design and versatility, and it was also the fastest performer in its class.

The IBM ThinkCentre A50, in its ultra-small case, is a well-designed no-frills PC that accommodates a modest amount of expansion – it includes a single PCI slot – in a confined space, with an easy-to-open case that requires no tools. The A50 uses the same optical drive model that IBM notebooks do, but you have to open the case to remove the drive.

Gateway's E-4300 4-Bay SB nicely balances small size and expandability: Placed horizontally, the case is just four inches tall, yet this model is the only system we looked at that contains a floppy drive and an open 3.25-inch drive bay. You get two PCI slots (although one is blocked by the power supply) and a single PCI Express slot; however, you can't use standard-size PCI cards in them – only the smaller, low-profile PCI cards will fit.

At just 3 inches high (or wide, when sitting on its side), the HP Compaq Dc7100 Ultra-Slim Desktop is the thinnest system in this group. However, it still sets aside some room for expansion, offering plenty of USB ports (two on the front and six on the back), plus space for a single PCI card inside the case.

All together now

All-in-one systems – LCD monitors equipped with built-in PCs – have been around for years, but typically they've been relegated to niche roles (such as in point-of-sale terminals, kiosks, and other small spaces) because they're difficult to upgrade and often lack power. That could change, however, as PCs migrate into kitchens, living rooms, and other places where a separate box is undesirable. And thanks to faster, smaller computer components, even an all-in-one is powerful enough for most computing applications.

All-in-one systems cost more than other small PCs, because their design is more complex. They offer no expansion options beyond USB and FireWire ports, but the models we examined are more varied than their predecessors. For example, one unit has a touch screen, and another boasts a built-in TV.

The Gateway Profile 5.5C has an innovative all-in-one design, with a wedge-shaped case. The Profile is the only machine we tested that builds the PC into the foot, not the back, of the monitor. As a result, the optical drives and the media reader are easier to reach, and the system is more stable because all of the weight is in the base. None of the components in the case are user-accessible or upgradable, however; and since the stand doesn't swivel, you can't angle the screen to the left or right without rotating the whole thing. You get plenty of USB and FireWire ports for connecting external devices, though.

The Sony Vaio VGC-V520G TV-PC is a pure entertainment buy. It's designed to serve as a living-room PC, with a built-in TV tuner, a remote control, and a dual-layer rewritable DVD drive behind the 20-inch wide-aspect LCD screen. This media machine doesn't use Windows Media Center Edition, but rather Sony's revised version, which improves on Microsoft's in a couple of ways. The system has no HDTV support, however. Because the screen runs at a resolution of 1,024-x-768, icons and text look overly large; as such, the system is best suited to the living room. A 17-inch model is also available. This Vaio was both the fastest and the most expensive of the small PCs we looked at.

The sleek, silver MPC ClientPro 414 is another business all-in-one unit built around an attractive 17-inch LCD. The monitor looked equally good when displaying text or DVD movies, but like most all-in-ones, the ClientPro has no room for internal expansion, though six USB ports and a FireWire port sit on the underside of the case. We found that the speakers failed to play back high-quality movie sound adequately, but they are fine for playing music CDs.

UK manufacturer Pelham Sloane markets its PS1500M primarily to integrators – outfits that then customize the unit for home automation (heating, cooling, security, and so on), mobile hospital carts, kiosks, and other uses; but the system is also available from Home Depot. The computer, with a Pentium M laptop processor, can sit on a desk, mount onto a wall, or attach to an optional arm. The machine remains a niche product, however, unsuitable for general computing because of its relatively high cost and comparatively low performance. Its steep price is attributable in part to its touch-screen display – a feature that makes it suitable for specialized applications.

Break the tower habit

Small PCs won't kill off the ubiquitous tower machine just yet, thanks to their lack of power and upgradability, but they make sense for many computer users. They have enough speed, reliability, and features to handle both home and business use. Because they don't take a brute-force approach to cooling, they produce significantly less noise than many towers. And if you already have a tower, a small box can be ideal as a second system.