you can do a system state backup with NTbackup, if you are running XP Home then you will need to install it off the CD, it is somewhere in the support folder.
after you install it take it out of the wizzard mode then work with it.
with a system state backup it will backup all of the registry, also all of the contents of the system32 folder, this means you will have all of your updates.
there is a lot more information on the MS KB about the system state backup, but don\'t limit the search to just XP.