Microsoft announced Tuesday that it would post the release candidate of Windows XP Service Pack 3 to its download site.

The move marks the first opportunity for all users of the six-year-old operating system to try out its final upgrade. Previously, several thousand users were given access to test builds of SP3 only by Microsoft's invitation.

According to a company spokeswoman, the version that debuts Tuesday, dubbed a "release candidate" to note progress from earlier betas, will be available from the Microsoft Download Center. She was unable, however, to say when the service pack would post to Windows Update so users can download and install it with the company's update service.

The final version of Windows XP SP3 remains slated for delivery sometime in the first half of 2008, the spokeswoman said. She also warned off casual users from trying the preview. "As this is a release candidate, we strongly encourage only those who are comfortable installing prerelease code to download Windows XP SP3," she said.

Recently, Microsoft has been downplaying the significance of Windows XP SP3. In a white paper posted to its Web site last week, and also Tuesday, the company praised Windows Vista at XP's expense, reminding users that "Vista provides the most advanced security and management capabilities of any Windows operating system."

"Windows XP SP3 does not bring significant portions of Windows Vista functionality to Windows XP," the spokeswoman said.

According to the white paper, the Download Center version of XP SP3 will weigh in at about 580MB; the version downloaded and installed via Windows Update, however, will be much smaller, typically around 70MB.

Windows XP debuted in October 2001 and was last updated as SP2 in August 2004; SP3 will be the final major upgrade of the operating system.