Alternatively, you could have just kept the old card and used only video modes which work. At the risk of the whole computer being damaged

It seems you were right, Mr Creosote.
Another graphics card solves the problem, while new drivers don't.

Tuss: it beeps like it always does

Does the computer beep at all when it boots up? Sometimes the manufactures use that as a debugger when the computer doesn't start.

Wild guess: The video card is defective and can't display certain screenmodes.

First, i'd like to make a statement:

I HATE WINDOWS XP!!!

Thank you. Now, here's what happened.

A friend of mine couldn't solve a computer problem, so he asked me(as usual). However, this time it wasn't the usual type of easy bug.
The computer (P4 2GHz, 256 MB ram, Geforce 4, Win XP Home) would go to BIOS normally, but then give only a black screen instead of the login screen.
I tried safe mode, and that works. Lask known working settings didn't, and neither did system restore. I started Spybot S&D 1.3 beta rc 4 to check what was starting and what wouldn't.
I found that winlogon.exe was started from the (impossible) path "/??/C:/windows/system32"
Aha, bug found, I thought.
Then, after a virus check, I tried getting Knoppix
to work. It works, but also (NOT NORMAL) shows a blank screen during most of the startup sequence.
Does anyone have an idea how to repair this?