Boot off the Windows Vista DVD and then click Next at the first prompt:
You should see a link in the lower left-hand side for “Repair your computer”
The installation process will attempt to find your Vista installation directory. If there’s more than one in the list, select the correct one and then use the Next button again.
Click the first link for “Startup Repair”
The installation process will repair the system, and then prompt for you to reboot.