Two weeks after it last handed a new build of Windows XP Service Pack 3 (SP3) to several thousand invitation-only testers, Microsoft posted that version for public downloading.

"We're broadening the availability of the release candidate in order to receive further user feedback prior to the release of Windows XP SP3," a company spokeswoman said. "Windows XP SP3 RC2 will be made publicly available Tuesday at 5:00 pm Pacific time (US)."

Earlier this month, Microsoft seeded Release Candidate 2 (RC2) with the 15,000 or so testers who had been working with SP3 for several months. At that time, the company said nothing about taking the version public.

This is just the second time that all Windows XP users have had the chance to try out SP3, the last scheduled major update to the six-year-old operating system. The only other public posting was of SP3 RC in December.

Microsoft, however, this time followed precedence. Earlier this year, it did the same thing -- sent code to its beta testers, then released it to all comers -- during the run-up to finalizing Windows Vista Service Pack 1. In fact, although Microsoft has stuck to the vague schedule of delivering XP SP3 some time in the first half of 2008, Vista's pattern may signal that the XP service pack release is imminent.

Microsoft seeded the RC Refresh build of Vista SP1 on January 11, then two days later posted it for public download. Twenty-two days later, it called a wrap on SP1, saying the code had met its RTM (release to manufacturing) criteria.

Another Vista indictor -- the posting of revised release notes just days before SP1 went RTM -- may also hint at XP SP3 be finished sooner rather than later. This week, Microsoft published the first version of the XP SP3 release notes to its Web site.

Once SP3 ships, the next major milestone for Windows XP is June 30, when the popular operating system is slated to fall off the reseller and retail availability list.

Microsoft is delivering SP3 RC2 via Windows Update, but users must first download, install, and run a small registry hack available from the company's Download Center. The hack allows the PC to "see" the SP3 RC2 update on Windows Update. Microsoft also told users to uninstall December's RC version of SP3 before applying this latest.