What should I do if XP fails to start after upgrading to Vista?

When you try to boot after failing to upgrade to Windows Vista, but you can't enter Vista xp system, you can't enter Vista, but display a black screen and a blinking bottom line.
How to solve this problem when you have this problem? Take a look at the following steps:
1, use the windows xp CD to start the computer.
2, in the "Welcome to the installer" screen, press R to repair Windows.
3, log in to the windows xp installation to be repaired.
4. Type fixboot and press Enter.
5. Type y and press Enter to confirm that you want to write a new boot sector to the system partition. When you do this, the following message is displayed:
A new boot sector was successfully written.
6. Type exit and press Enter to exit the recovery console.
7, restart the computer, and then complete the Windows Vista upgrade operation.