H: Blank screen after powerup

[Alpine]

I restarted my computer this morning and now all i get is a blank screen.
The mobo screen comes up after i power up, there's a little underscore or dash that flashes in the top left of the screen, and then the screen goes blank (sometimes accompanied by a little flicker).
I can enter the bios and make changes but after that i still get a blank screen. I've tried safe mode and the other modes from the options screen, but all i get is a blank screen afterwards. I've tried booting from my WinXP CD and i still get a blank screen.

I've tried using another videocard but i get the same problem.

It's a pretty old computer, P4P800 ASUS MB, P4 2.8Ghz, ATI 2400HD videocard.

Does anyone have any ideas on what might be the problem?

[John]

Try reinstalling the video driver to see if that helps, then
Try reinstalling Windows to see if that helps, then
Junk the computer

[underscore]

reinstall/repair windows? sounds like your bootloader is messed up, as I get the little underscore blinking for a second before it asks me to pick 7 or Vista