Microsoft has delivered a release candidate of Windows Vista Service Pack 1, set to ship early next year, to about 15,000 invited testers.

A new build of SP1, Windows Vista SP1 RC Preview, was made available to testers on Wednesday, according to a spokeswoman. "The RC Preview incorporates feedback from users in our beta programme, including changes to the set-up and installation experience," she said.

The first beta of SP1 was seeded to approximately 12,000 testers about seven weeks ago, after considerable ship date speculation and several months of uncertainty whether the company would even produce its usual service packs for the new operating system.

"We will release an RC of SP1 to a broader group of testers soon," the spokeswoman said. "And we are targeting the first quarter of 2008 to release SP1 to manufacturing."

Microsoft, however, did not respond to other questions, including whether a pre-release version of SP1 would be offered to all users.

Vista, which has received its share of criticism from users and reviewers, has been deployed by relatively few big businesses in the 12 months since it was released. A new survey by Forrester Research, for example, says that only 7 percent of US and European companies will have started deploying Vista by the end of the year; climbing to 32% by the end of 2008.

Although Microsoft is obviously extending the service pack concept to Vista - a fact that was unclear for several months after the operating system shipped to consumers in late January - it's also made much of several recent performance, stability and reliability updates that have been pushed to all Vista users. The most recent was added to the Automatic Updates line-up offered via the Windows Update service on Tuesday.

"Continuous improvement is the name of this game," said Nick White, a Vista programme manager, in a posting on a company blog that described the three updates provided to all users this week.