Hold on, do you want to repair? without loosing your files?
or do you want to reinstall, and wipe out everything that is on the computer?
Answer that question first, before doing anything. Back up all your personal files first, if you can.
If you want to repair only, see Tom's XP forum, fifth post from the top, there are directions.
put the disk in the drive
start the computer and hold down F8, wait for the boot menu
select: boot from CD, using your up down arrows, push enter
follow the install directions as they appear on your monitor.
usually works OK.