You can try this with the Windows XP disk. It will most definately repair XP boot manager, not sure about VISTA.
1) Configure your computer to boot from the CD drive.
2) Insert your XP CD and boot from it.
3) You'll see some files being copied, then you'll be presented with a choice of installing or
repairing an existing installation. Choose Repair.
4) You'll be asked which XP installation you want to log into. Enter 1. There is usually only
5) You'll be prompted for the Administrator password. For Home, the default password is blank, so just hit Enter. For Pro, enter the same password you did during setup for the Administrator account (this is not the same as the password for an Admin level account. It must be the Administrator account password).
6) At the C:\Windows prompt, type FIXBOOT. You'll be prompted to confirm. Do so.
7) When FIXBOOT is finished, remove the XP CD and type EXIT and the machine will reboot.
Reconfigure your computer to boot from the hard drive if necessary.