You can now use Windows Update to install SP3 to a single Windows XP system: no matter how many updates your system was waiting for, Microsoft has moved SP3 to the top of the list.
Windows XP SP3 upgrade packages are also available for download from the Microsoft Download Center. If you prefer to install from CD, grab the ISO-9660 CD Image File. The US English version weighs in at 544.9MB. If you need software to burn an ISO image to CD, try the freewareImgburn program.
Rather download an .exe file you can use on multiple PCs? Download the Network Installation Package, which takes up 316.4MB in its English-language version.
Before You Install SP3, Watch Out for Gotchas and Workarounds
Any time there's a major update to Windows, there are a few issues that can cause trouble. Here's the list for SP3:
[Corrections below are based on version 1.1 of the Release Note for SP3; corrections as of 5-7-08]
Have you already updated your XP SP2 system to IE7 or a beta version of IE8? You need to uninstall IE7 or IE8 beta before you install SP3 (SP3 includes IE6) and reinstall your choice of IE7 or IE8 beta after updating to SP3. Need to uninstall IE7? See KB 927177 for help. Additional tips are available here. It's ironic that IE7 was still being offered by Windows Update as a high-priority upgrade for XP SP2 users as of mid-afternoon May 5th! Uninstalling IE8 beta 1 is easier: just use Add or Remove Programs (see KB 949787 for details).
If you want to uninstall IE7 or IE8 beta, you need to remove them before installing SP3, then reinstall them after installing SP3. If you don't, you can't uninstall these versions of IE after upgrading to SP3. For help with uninstalling IE7, see KB 927177; for help with uninstalling IE8 beta, see KB 949707 (a hat tip to reader zeringue for pointing out this correction).
Running the Windows XP SP2 Multilingual User Interface Pack? It doesn't work properly with SP3. You need to install a version updated for SP3.
KB 937468 (a prerelease version of the Remote Desktop Client v6.1) should be uninstalled before installing SP3, but there's a workaround if you didn't uninstall it first.
Changing regional and language options after installing SP3 can cause VBScript to use the wrong format for outputting regional and language-specific data.
If you have Windows XP Media Center Edition 2002, be sure to update it to SP2 before installing SP3.
If you use SQL Server 2005, SQL Server 2000, or programs that incorporate SQL Server, such as Microsoft Dynamics RMS, Redmond has a hotfix for you - and you should apply it to your Windows XP SP2 system before you upgrade to SP3 (if you're upgrading to SP3 from SP1, install it as soon as you upgrade to SP3). Learn more about it at KB 951937.