XP, Video Card, Driver, Hard drive, fie!

I'm a beginner, so please bear with me.

I have a Dell XPS 400 running windows XP media center edition. A few days ago, my screen started showing about 1" square blocks of miscolored, flashing checkered patterns in random places while playing video games. I ran a virus scan, it came back clean.

I downloaded a new driver for the videocard - Geoforce 6800. Shortly thereafter, maybe 3 or 4 full rebootings of windows later, the screen was full of previously mentioned video gliches, even while the the windows loading screen was up. Now it just goes black after the windows screen is done.

The hard drive seems to be at peace while it's black - all I hear is the fan. I've managed, through the garbled letters, to find what I think is safe mode once or twice, but as soon as I log in as a profile it takes me to cmd.exe - I've got nothing. The version of XP is apparently 5.1.2600, according to that screen.

Hmmm. Just tried again, now there are flashing smily-faces in a few spots. I can't imagine that is a happy development. Fie!

Any and all help is appreciated. Is less information better, or is this not enough?