To work around this issue, use the Windows XP Recovery Console to write a new boot sector to the system partition. To do this, follow these steps:
1. Use the Windows XP CD to start the computer.
2. At the Welcome to Setup screen, press R to repair Windows.
3. Log on to the Windows XP installation that you want to repair.
4. Type fixboot, and then press ENTER.
5. Type y, and then press ENTER to confirm that you want to write a new boot sector to the system partition. When you do this, the following information appears:
The new bootsector was successfully written.
6. Type exit, and then press ENTER to exit the Recovery Console.
7. Restart the computer, and then finish the Windows Vista upgrade operation.
8. Reinstall the third-party boot manager program.