Microsoft has released Windows Vista Release Candidate 1, putting the new operating system into its final round of bug fixes and tweaks before going on release.

Windows Vista RC1 is available to TechBeta and Technology Adoption Program (TAP) program subscribers, but MSDN and TechNet subscribers will have to wait until later this week. Microsoft has said since March that Vista will be available to business users in November and consumers in January 2006.

With the last major upgrade to Windows, XP, it was about two months between RC1 and the product's release to manufacturing. If Vista follows a similar schedule, with RC1 out Friday, the release to manufacturing - and a release to business customers - could happen as soon as early November.

OEMs typically need about six weeks to get the OS installed and tested on PCs before putting those machines on the market. If Windows Vista makes it to manufacturing by the end of November, it could hit its January release date for consumers.

Amazon.com is already taking pre-orders for Vista, and has listed availability as 30 January.

In a letter to TechBeta subscribers on Friday, Jim Allchin, Microsoft co-president of the Platforms and Services Group, thanked them for their support and feedback, and mentioned several improvements to the OS that have been made since Beta 2 was released in May.

"We’ve made some UI [user interface] adjustments, added more device drivers and enhanced performance," Allchin wrote in the letter. He added that Microsoft will continue to work on Vista's "application compatibility, as well as fit and finish" until its release to manufacturing, which is yet to be determined.

Allchin also advised independent software vendors to use RC1 to certify its applications to run on Windows Vista, and asked for more feedback from testers to improve the OS before its final release.