Vista Wont Boot Unless in Last Known Good Configuration

I've seen one other guy (zephyrhills) who was having the same problem as I.
I have installed vista onto a second partition and when I boot my pc it gives
me the screen to choose OS. Upon the first time choosing Vista it worked
great!!! I wanted to restart to xp (and did) then went back to vista (after
making no changes). It told me that it didn't shut down correctly and gave
me the choicies (safe mode... last known good config... normally) so i chose
normal. It started to boot, but then hung up and eventually my monitor went
off because of no signal from the pc. I'm using DVI btw. So anyway, i
restart again and do last known good config. Works great!!! so i install the
drivers thinking this is why, and restart to vista, same thing!!! Wont boot
normally, and only works with last known config. When i choose last known
good config, it reverts back to the pc's original state (no drivers or
anything) so i'm in this non stop loop. Any help would be apprecatiated.
Specs below:

A8n SLI deluxe mb = amd 64bit 3500+ SATA 190 gig maxtor hard drive, 1 gb ram.

thanks