if you can get into windows, have your tried re-downloading the updates? if that fails, backup your important files (put hd in another pc or something) then wack in the XP cd and get it to format the computer and install again - thats if you have a OS cd. Or have you tried the "go back" feature (or what ever its called) in XP? allowing you to restore your pc to a previous date (obviously before the update).
Hope this helps
Dom