All right, here is the deal. I got a new video card (ATi Radeon 9800 Pro) and it's being bad. I insert it. Attach the power cable and turn the PC on but get nothing. A fast memory testing screen and when it completes the PC locks. Can't do anything except reset or turn the computer off.

My money is on the PSU which might be borked but thought I'd post my problem and see if someone can come up with something.

what card did you have before that?

nVidia GeForce4 MX 440

I've tried anything basically, concerning BIOS. Changed Halt to not halt on errors, changed Init Display First from PCi to AGP. But seems that when I take the old video card out and put in the new BIOS resets itself to default.

Edit: Forgot to mention. The card doesn't seem to be dead because the first time I inserted it I forgot to connect the power cable to it and on the screen a text appeared saying something in the lines of "There is no power cable connected to your ATI RADEON 9800 PRO, make sure etc etc"
Plus the fan works.
Plus, when I connected the video cable to the on-board video out plug there was no signal meaning a switch had been made between the AGP slot and the on-board thing.

did you uninstall the nvidia drivers (and use nvidia driver cleaner?) before putting the new card in?

Well the drivers shouldn't even affect it as I don't even get to the Windows loading screen. It locks up even before that.

You should really always uninstall the drivers before changing card, especially from nvidia to ati (or vice versa). Uninstall the drivers then google and run "driver cleaner" in safe mode to completely remove the nvidia drivers, then put the new card in and see if it boots. If it does, live with the horrible display for a while and download the newest ati drivers for that card and install them =p