That sounds like something else has gone wrong. If the MBR, PBR, or the signatures are messed up the BIOS should be complaining. Check your hardware first.
Check this site out:
Multibooters - Dual/Multi Booting With Vista There is lots of good information on the XP and VISTA boot process and what can go wrong with it. There is also some information on cloning VISTA. This is hard as the new VISTA boot process does not play nice with imaging techniques. Lots of people are having problems so do not feel bad.