Constantly Reboots After Install

>I decided to try out Vista, and since I had no intention of an upgrade
> or even possibly messing up my existing XP install, I decided to do it
> in a spare drive I had laying around. So I disconnected my old
> drives, popped in the spare 60gb one, and let'er rip.
>
> Well the install went through pretty painlessly. It's actually a
> fairly nice installer compared to all the previous ones, I have to
> say. I got my user account created, it checked my system performance,
> etc etc. Didn't take all that long until I was sitting at the login
> screen. Welp, I typed in my password, it gives a message that it's
> starting to log in, but then it says "Shutting Down", and reboots.
> And reboots. And reboots. Forever.
>
> I can't even get the splash screen, it just reboots almost instantly
> and goes back to the initial system startup. The same thing happens
> when I reinstalled it all over again. Picking repair in the DVD
> options did nothing; it said it was fine. I did a System Restore just
> for the heck of it, no difference.
>
> Then I tried using bcdedit from the recovery console and adding in
> extra false boot configurations, so that the boot menu might stay up
> for some length of time. I even set another one of them to be the
> default. But it still constantly reboots. I can tap F8, or hold
> shift, or any of the things that used to do something in XP, and still
> nothing. I might see a flash of text from the boot loader, and then
> reboot.
>
> This is just really frustrating. I don't understand what the deal is,
> considering it rebooted two or three times during the install process
> just fine. It wasn't until I tried to login that it screwed up. I
> even pulled the network cable out the second time I installed it,
> because somebody suggested that it might think my copy was stolen or
> some such. Made no difference.
>
> Has anyone else experienced such a crazy problem?
>