if the error is about NTLDR being missing I have a solution.
Grab an install cd, doesn't matter if it is acquired from a shady source or not. Boot from the cd and look for the option to go to the recovery console. At the prompt, type in
fixmbr
and
fixboot
then reboot. Your computer should work after that.
Disclaimer: I haven't had to fix Windows in this way for a while. As far as I remember this is how to do it. Attempt at your own risk. If in doubt, make a backup. If not in doubt, get in doubt. I have no idea if this will work with Vista. This method will also wipe out any other operating system bootloader so beware.