As it promised, Microsoft has started sending Windows Vista users an update that identifies illegal copies of the OS installed with cracks that the company will disable when it distributes Service Pack 1 (SP1) in two weeks.

Last Thursday, Microsoft announced the update, which detects two common cracks used to activate pirated copies of Vista, and said it would hit Windows Update (WU) within a week. Users who have left Vista's recommended WU settings alone will receive the update automatically. Others, said Microsoft, must enable Automatic Update within Vista or manually call up Windows Update from the Start menu.

A document posted to the company's support site spelled out the details. Among other things, it promised that the 3MB update "does not affect the functionality of your operating system." That jibes with what Alex Kochis, senior product manager for Microsoft's Windows Genuine Advantage program, said last week when he announced the update.

If the update finds one of the cracks – the first, dubbed "Grace Timer," extends Vista's activation grace period, while the other, "OEM BIOS," mimics factory-floor activation – it pops up a notice alerting the user, but doesn't disable either crack. Users whose PCs don't have the cracks will not see the pop-up.

Once installed, the update cannot be removed, as it doesn't appear in the "Uninstall or change a program" list in Vista's Control Panel.

The update is one of the last steps Microsoft's taking before it offers SP1 during the middle of next month. The service pack will kill the Grace Timer and OEM BIOS cracks, a move that in turn prompts the operating system to start showing messages informing users that they're running a bogus copy and persistently nagging them to pony up for a legitimate version.