After doing a repair install, although most of your apps, drivers and user settings will still be in place, all windows updates, and possibly some utilities will need reinstalling anyway
Boot from the XP CD and at the first menu, press Enter to "setup windows xp now"
accept the EULA with F8, and it searches for existing windows installations
select your windows install from the list (if there's more then one) and press "R" to perform a repair install
if no windows installations are listed, or it does not give an option to press "R" to repair, you will have to do a full install