Incomplete graphix on screen and slow com...

What is the wattage of your PSU? You should have, minimum, a 300W psu installed. that "could" be the problem.

Are you able to get through POST boot? If so, can you load into safe mode? I would completely uninstall the drivers and reinstall again. I assume you're using XP, and with XP, it's always best to remove old vid drivers before installing new. Safe mode is the best place to do this, as it only loads the very basic vid drivers.

If you cannot get to safe mode, pull your card out, blow air in the AGP/PCI slot it uses. You may have some dust down there preventing a good connection.

Pop the card back in, load into safe mode if you can, uninstall/reinstall the drivers. If that is a no go, replace the card. Sometimes you DO get bad cards off the shelf.