After I installed SP3 twice, on new reinstalls (both XP Pro SP2 new installations), the Windows Update would no longer update (it would download new files but fail.) I searched for the error message entering it in search engine verbatim with quotes and finally found sites leading me to check my update.log I then searched for the error message which I found at the end of that log and found sites stating the update manager had to be reset (I forgot the exact commands, it was about 4 line commands to enter in the cmd/dos prompt.) That reset worked and new updates installed correctly.