Load your Windows XP installation CD, and then restart the computer, this should give you option of pressing any enter to start the CD. If this is not the case, then enter your BIOS by pressing F2 before the system has fully booted, and change the boot priority order which is found under the BOOT section of the BIOS menu. Make the CD-ROM the top of the list.
Save the settings and then restart the computer, the CD should now load, whereby you will need to press F8 when a blue screen loads. Press R which should load the Recovery Console, and then choose the Startup Repair option.
Hope this helps,
P.S. Try this website for more information - How to repair the boot sector by fixing the master boot record (MBR) in Windows XP | Computer Tips