Sometimes it's just not an option to reformat a hard drive before reinstalling Windows XP.
If you have data that you can't backup or programs that you can't later reinstall, reinstalling Windows XP without reformatting is a must.
A repair installation will install Windows XP again, over top of the existing installation that you're currently having problems with.
While a repair install is designed to keep all of your data and programs intact, I highly advise that you backup everything you can before undertaking a repair installation. If something were to go wrong during the reinstall, it's possible that data loss could occur. Better to be safe than sorry!