Microsoft posted the release candidate of Windows XP Service Pack 3 to its download site Tuesday, giving most users their first opportunity to try out the upgrade.

Several thousand testers had tried SP3 before, but only by Microsoft's invitation.

According to a company spokeswoman, Windows XP SP3 RC (release candidate) will be available only from the Microsoft Download Center. Unlike Vista SP1, which debuted last week, XP SP3 will not be soon added to Windows Update. In fact, the spokeswoman seemed to say SP3 wouldn't be offered to users via Microsoft's update service before the service pack is finished next year.

"XP SP3 2ill be added to WU [Windows Update] in 1H '08," she said in an e-mail late Tuesday.

The download weighed in at about 336MB, but when SP3 is offered through Windows Update, the installation file will be much smaller -- around 70MB.

Even though the release candidate can be installed by anyone running XP SP2, Microsoft warned off casual users from trying the preview. "As this is a release candidate, we strongly encourage only those who are comfortable installing pre-release code to download Windows XP SP3," said the spokeswoman.

She also confirmed that the final version of Windows XP SP3 remains slated for delivery in the first half of 2008.

Recently, Microsoft downplayed the significance of Windows XP SP3. In a white paper posted to its Web site last week, the company praised Windows Vista at XP's expense, reminding users that Vista boasted beefed-up security, for instance. The spokeswoman also chimed in. "Windows XP SP3 does not bring significant portions of Windows Vista functionality to Windows XP," she said.

That may be so, but according to a Florida performance testing software developer, XP SP3 is not only 10% faster than XP SP2, but more than twice as fast as Vista SP1, claims that Microsoft disparaged within days.

XP SP3, in fact, is the newest version of Vista's biggest rival, according to Forrester Research. US and European businesses will delay Vista deployment, Forrester analyst Benjamin Gray said a month ago, in part because of application incompatibility problems unheard of in XP. "That's causing a lot of XP shops to take a wait-and-see approach to Vista," said Gray then.