Microsoft has released Windows Vista SP1 RC Preview to 15,000 testers as part of its private beta program.

On Tuesday, the company released three patches that will be part of SP1 when it ships in the early part of next year.

Microsoft hopes to align SP1's availability with Windows Server 2008, which Microsoft plans to ship on or before its Feb. 27, 2008 launch event in Los Angeles.

Microsoft officials said the Vista SP1 RC Preview incorporates feedback from users that have been part of the beta program that began in September. The new features include changes to streamline setup and installation.

Microsoft said a broader group of testers would soon get the RC Preview, but it did not announce a specific date.

Over the years, SP1 versions of any Microsoft products have become a traditional milestone that some corporate users wait for before they even consider rolling out the software.

Uptake of Vista has been slow by corporate users, many of whom have standardized on XP and are reluctant to undertake another migration.

A study by Forrester Research shows that 52 per cent of respondents have no plans to roll out Vista or don't know when they might do it. The study shows 11% don't plan to start a roll out until 2009 and another 6 per cent are waiting until 2010.

Vista SP1 will include a number of bug fixes and performance enhancements but no new features, Microsoft said.

On Tuesday, Microsoft updated its set of Vista migration tools, including the Application Compatibility Toolkit (ACT) 5.0, Windows Vista Hardware Assessment (WVHA) 2.1 solution accelerator (formerly called Business Desktop Deployment), and Microsoft User State Migration Tools (USMT) 3.0.