think System Restore is the most useful program default in Win XP. It has saved my computer from the dreaded 5 hour reformatt, reinstall, redownload all your programs and updates. I get pissed especially when I accidently instally those pop-up ad programs and when you try to uninstall them, sometimes you'll need to use a uninstaller from the program website but they just won't let you d/l that uninstaller! I just restore and eveything is fine and dandy.
It's also great for hiding viruses, taking a large space on the Hard drive and slowing down the computer by creating random checkpoints.