If the Windows XP Bootloader gets erased or corrupted, you can repair it without a complete reinstall. Below describes how to do this. More useful Windows tips can be found at WindowsInternals.com.
Disclaimer: Attempt this at your own risk, I assume no responsibility for any data lost.
Boot from the Windows XP CDROM in to the “Recovery” prompt. If you have not set an Administrator password, leave it blank. The Windows XP documentation (Help and Support) has a section on the Recovery Console if you’d like to learn more.
If you’re not comfortable using the Recovery Console, you may like to try software like XP Repair Pro which also works with Windows 7.