Well, anyway you do this is going to require the install CD, but you can use
it from two different places. You can boot to windows, insert the CD, and
then choose to do an "upgrade" install. The "upgrade" is the only option to
choose, if that is not available, you'll either completely go over windows
and lose a lot of your data and documents, etc or you'll end up with a dual
boot which you really don't need. The other option is the booting from the
CD. If you're having problems, I'd suggest castlecops.com as the best place
to go for really good advice and help. They are very good. Here is a link
with more info on repair installs.
http://michaelstevenstech.com/XPrepairinstall.htm
PS, if you manage to do a repair install, it may turn your firewall off.
Before you go online make sure to TURN ON the Win XP firewall. I've heard of
people picking up worms within minutes, especially on high risk networks.
(i.e. school networks)