Usually gets anywhere from 100-400 meg of HD space back.
CCleaner is a good one to run as well and remove all the extra uninstallers for patches you are not going to uninstall...
Too bad vista/win7 dont have something like that (winsxs). Win 8 they added something similar into the dism tool.
Just keep in mind next year there will be 0 new patches for XP. Then pretty much any time MS patches the newer versions new viri will show up for it. As many times the bugs have been in there for a long time.
Only reason I keep vista around is because the newer media players corrupt mp3s.