Microsoft released the beta of Windows Vista Service Pack 1 to a private group of testers, taking one step closer to the anticipated official launch in the first quarter of next year.

In late August, Microsoft for the first time offered a release schedule for the long-awaited SP1. At the time, the software giant said the beta would become available this month to between 10,000 and 15,000 testers.

One beta tester, Brandon LeBlanc, author of the Windows Experience blog, has already written about his experience with SP1.

In addition to a couple of minor user interface changes, he has noticed improvements to the overall responsiveness of his PC and laptop, which both got the beta software. He particularly noticed that resuming activity after the machines hibernate and copying files from one directory to another work faster.

LeBlanc also reported improved battery life on his laptop.

SP1 also appears to have solved some wireless networking issues he had, which caused his computer to lose connectivity often, especially after the laptop was hibernating. Since he installed SP1, that problem has gone away, he said.

Microsoft had promised that SP1 would include a speedier resume function following hibernation. It also said the update would include improved performance of Internet Explorer 7, particularly for Web sites running AJAX.

Microsoft will distribute SP1 via Windows Update, its automatic update service. Alternatively, corporate administrators can use a standalone version of SP1 in conjunction with other programs they may use to distribute software to computers.

Also on Monday, Microsoft released the initial Release Candidate (RC0) of Windows Server 2008, code-named Longhorn. That means customers can download and try out the latest version of Windows Server 2008, including for the first time a preview of Viridian, Microsoft’s virtualization technology. The release of the RC0 means that development and testing are progressing and that the server code is entering the final stages of testing, Microsoft said.

Last month, Microsoft pushed back its forecast for the release of Windows Server 2008 from this year to the first quarter of next year.