Repair the Master Boot Record in XP
Use the fixmbr command, available in Recovery Console, to repair the Master Boot Record on an XP machine. If a computer won’t boot or sustained some damage due to a virus or other cause, this may resolve the problem.
Boot from your Windows XP CD and when prompted, press R to access the Recovery Console.
1. When you reach the command prompt, type the following and then press Enter.
2. 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.
3. 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.