Microsoft could release the first beta of Windows Vista Service Pack 1 any day now, with a final release planned by November, according to sources close to the company.

Microsoft has been mum about exactly what will be in Windows Vista Service Pack 1, a roll-up of updates, many of which are pushed out automatically through its Internet-based update services. The company also has not said when the software will be released. The company has said only that a beta will be out this year.

WinBeta.org blogged this week that Microsoft's Windows Driver Kit (WDK) team sent an email to beta testers telling them that a new build of the WDK was being released to them to coincide "with the recent OS beta release for Vista SP1 Preview," hinting that SP1 beta is soon to be sent to testers.

Others speculate Microsoft might release the SP1 beta today, when it will report its fiscal 2007 fourth quarter and year-end financial results.

One source close to the company said in an email he had been told by insiders that a beta of SP1 would be out by now, so an imminent release is highly likely. And another said in an instant message that testers report SP1 will definitely be out this week, with a final release coming in November or December.

At Microsoft's Worldwide Partner Conference in Denver two weeks ago, Mike Sievert, corporate vice president of the Windows business group at Microsoft, toed the party line on Vista SP1 and said little about when it would be released or what would be in it, and hinted that Microsoft's own plans for the service pack are still in flux.

Sievert also downplayed the update's role in when some Windows users will upgrade to Vista, though it's widely known many users - especially business and enterprise customers - are waiting for SP1 before replacing Windows XP with Vista.

"Most companies are still in the evaluation phase," he said in an interview. "Service pack or no service pack, they're still looking at what they will deploy."