WinXP keeps rebooting at reboot

1. Boot to your Xp cd
2. Follow the same steps you would use to install Xp.
3. When you get the "Select partition to install Xp on" message, select the partition that Xp is already on and click OK.
4. You will then be presented with a message along the lines of "Xp already detected on this partition, would you like to repair?" click Yes or Ok to continue.
5. That will write a brand spanking new MBR, and fix any problems with system files also.

(courtesy: http://www.computing.net/answers/windows-xp/winxp-keeps-rebooting-at-splash-scr/75642.html)