Microsoft has released a new build of Windows Vista Service Pack 1 (SP1) for the second time within a fortnight.

It has sent the new version, Vista SP1 Release Candidate (RC) Refresh 2, to its invitation-only group of approximately 15,000 testers. "This group includes corporate customers, consumer enthusiasts, software and hardware vendors, and others," a company spokeswoman said. "The code is not available for public download."

Microsoft made the same claim two weeks ago, when on 11 January it released SP1 RC Refresh, saying then that it would keep the build private. Two days later, however, it posted the refresh to the Windows Update service.

Microsoft has repeatedly said it expected the release of Vista SP1, the long-awaited first major update to be in the first quarter of the year. However, after a Taiwanese report that claimed 15 February would be SP1's day, talk intensified of the service pack's imminent release. When asked about the rumours, the Microsoft spokeswoman answered: "The final release date is based on quality, so we will continue to track customer and partner feedback from the beta program before setting a final date."

Microsoft also issued a new build of Windows XP SP3 this week. Tagged as SP3 RC Refresh 2, the update went to the same group of 15,000 testers, and like Vista SP1, was made available via the company's Connect beta site.

An earlier preview of Windows XP SP3 was released to the public about a month ago, and is still available for downloading. XP SP3 is scheduled to go final in the first half of 2008.

Windows XP SP3 has consistently been given shorter shrift by Microsoft. Unlike its public relations efforts via company blogs on Vista SP1, the company has made little effort to broadcast XP SP3's features or changes. In fact, some analysts have noted that the biggest barrier to Vista adoption is the continued devotion of business customers to Windows XP.

In November, for instance, Forrester Research analyst Benjamin Gray said: "Vista's biggest competition isn't Apple or Novell or Red Hat; it's Microsoft itself, it's XP."