I just installed SP3 for XP and realized a couple days later that all windows updates were failing during the installation process.
I later found out that SP3 lays down a few new files, but does not seem to register the new DLL's...Thanks MS! Maybe this is a side effect of Bill Gates leaving the company??
Anyway, in order to fix this anomaly, just run these commands in a batch file.
regsvr32 /s wuapi.dll
regsvr32 /s wuaueng1.dll
regsvr32 /s wuaueng.dll
regsvr32 /s wucltui.dll
regsvr32 /s wups2.dll
regsvr32 /s wups.dll
regsvr32 /s wuweb.dll
After that, all should work fine. Hopefully I don't experience anymore issues with SP3, because so far, it has only done me evil.