Microsoft will ship the final prerelease version of Windows Vista on Friday and, unexpectedly, will name the new version Release Candidate 2 (RC2). (Microsoft had stated earlier that it wouldn't ship an RC2 milestone release of Vista.) But don't let the name fool you. RC2 is really just a standard interim build of Vista, but unlike RC1, it won't be distributed to millions of users.

The RC2 build is 5743, and Microsoft is currently testing it internally. As with the previous interim build, 5728, RC2 will go to beta testers, Microsoft Developer Network (MSDN) and TechNet subscribers, and a random, limited number of Consumer Preview Program (CPP) members.

Although Microsoft wouldn't confirm the specifics of the release, the company did corroborate that it's planning an upcoming interim Vista build. "As we have previously discussed, Microsoft does plan to deliver more interim builds of Windows Vista before releasing the final product," a Microsoft spokesperson told me. "The exact name of the build and the date of its release have not been announced."

This is the last build Microsoft will be making available to the public prior to release to manufacturing (RTM). The RTM--which had been expected to be "on or before October 25"--has been adjusted slightly. Now, the Vista launch window begins October 18, when Microsoft will begin testing the final planned build for RTM. But Microsoft doesn't expect to finalize the product until sometime between October 25 and November 8.

Internal documentation notes that each daily delay after October 25 comes at a price, however: For each day past October 25, Microsoft will ship one less localized, language-specific version of Vista for the January launch. However, the company is prepared to do that to ensure that Vista's quality is high.

The internal documentation also notes that Vista still has more than 1400 bugs. Microsoft's internal processes require this bug count to drop to 500 or fewer before the product can go into escrow for RTM. By comparison, Vista had more than 2470 bugs on September 22.

In related news, Microsoft will be changing the product keys it uses for Vista after the product hits the RTM escrow phase. Consequently, users won't be able to use prerelease Vista product keys to install the RTM version. My understanding is that this change won't affect the RC2 release.

Update: Since this article was first written, Microsoft has had to advance the RC2 build to 5744.16384.061003-1945. The company still plans to ship RC2 to testers on Friday.