Repairing the master boot record on your Windows XP system is accomplished using the fixmbr command, available in Recovery Console. This is necessary when the master boot record has become corrupt due to a virus or some kind of damage.
Follow these easy steps to repair a damaged master boot record in Windows XP.
When you reach the command prompt (detailed in Step 6 in the link above), type the following and then press Enter.
The fixmbr utility will write a master boot record to the hard drive that you're currently using to boot into Windows XP. This will repair any corruption or damage that the master boot record may have.
Take out the Windows XP CD, type exit and then press Enter to restart your PC.
Assuming that a corrupt master boot record was your only issue, Windows XP should now start normally.