Download Windows XP SP3 Release Candidate
First up, some members of the WinMatrix forum have posted a download link that seems to come straight from Microsoft's servers. In the event that this link stops working, you can also check out the BetaNews FileForum, where there's a hosted copy of the service pack. We generally wouldn't advise anyone to download an operating system upgrade from any site other than the official developer's page, but then again, if you're that concerned about protecting your computer, you should probably just wait for the final release anyway.
Release candidate builds are generally more stable than beta versions of software, but if Microsoft was ready to release the final build, well, that's what they'd be releasing then, isn't it?
The whole file weighs in at 336MB, but the updates are cumulative. That means if you've already downloaded and installed a bunch of hotfixes to your Windows XP SP2 system, you've already got many of the updates included in SP3. So the only way to know how much space this update will eat up on your hard drive is to install it.
[via CyberNet and BetaNews]