Sometimes it's just not an option to reformat a hard drive before reinstalling Windows XP. Most of the time this is because you have important files that you haven't backed up and erasing them is simply not something you're OK with doing.
While newer versions of Windows have more extensive repair and recovery options, it seems that just about every major problem with Windows XP requires a brand new, destructive reinstallation process.
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.
"How Do I Reinstall Windows XP Without Reformatting?"
The most effective way to reinstall Windows XP without reformatting your hard drive is to perform a repair installation of Windows XP.
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!