Black screen, blinking underscore + yellow screen

Hi everyone!
I have a crazy problem with my PC:
It used to work fine yesterday, but it would hang up today while booting showing a blinking underscore on a black screen.
Same thing with Windows Installation: After loading the essential drivers, he would show me the blinking cursor forever.
After reading several posts on this issue, i decided to update the BIOS and reset everything there - which has fixed at least the blinking cursor on booting (not on reinstalling Windows, though). I can see the booting screen perfectly and in correct colors. But afterwards, screen gets yellow and the machine locks up.
It's not a HDD issue, i tried other ones in the machine.
It's not the graphics card, I just got a brand new ATI and same thing happens with other cards I tested in my machine.
Basically, that's the two devices I would have suspected to be causing this trouble.
Isn't that kind of weird?

If anyone has experienced similar problems, I would be pleased to receive your helpful responses!

Thank you a lot,


Daniel