(Bored at work and happy to help :) )

Just to confirm, you can see Windows (XP or Vista?) starting normally (wavy flag and all) until you get to the login screen? Can you describe the trippy mess?

You see a black screen at login with the motherboard graphics: can you get into the BIOS using your motherboard integrated graphics? You won't be able to touch the drivers from BIOS, I'm just trying to narrow down the possibilities.

On the solutions front, do you have any kind of windows disc or a boot disc? Unrelatedly, do you have access to a graphics card that doesn't use the PCIe port (may or may not help, depending on previous answers)?