Constant reboot after video card driver installed

I recently install Vista and had to update V. Card drivers. I must have
installed the wrong driver b/c now all it does is reboot. I have already
installed this copy of Vista on the same machine in the past and went back to
XP. Now that Vista has been around awhile I wanted to reinstall it. If I run
the repair will it matter that I have activated it twice already? Or is there
a way to roll back the driver without repair. I've tried all prompts, all
safe modes, last known ever option reboots. Thanks