Computer won't reboot

Hi, I can’t get my computer to reboot after installing W/vista.
I had xp, installed vista cd then tried to restart and now wont get past start up. It say’s the application is missing or corrupt?
Computer was working fine, now won’t work at all!
Any advise would be great.:confused:

Erase the hard drive and reinstall Vista.
If you have a recovery CD set try that after you erase the Hdd.