FYI. If you are considering applying SP3 to Windows XP, it has been recommended that you use the full ISO CD image for the installation rather than the cut-down automatic update. Visit Microsoft and search for "SP3" and "Windows XP" as search terms to find the CD image. It is 544 MB in size.
According to IT sites and forums, it seems that the automatic windows update does not contain all the files that might be needed when some PC vendors ship strange or unnecessary driver files with their streamlined XP installation. Currently this seems to have caught out a number of HP users who have AMD processors.
I'm just about to burn and apply the update for SP3 since my PC has been a bit twitchy with frequent and temporary browser and explorer lock-ups ever since I uninstalled Symantec Internet Security. Fingers crossed that this update makes things better and not worse!
Regards,
Jovial.