Some Motherboard have a video chip built right onto them and I was thinking that maybe Windows was getting confused with two video cards (which isn't a problem when Windows is working properly). However, if you don't know if you have one means you likely don't since adding a second graphics card would have been a conscious decision you made at some point.
To check the BIOS you typically have to hold down one of the function keys F5, F6 etc. when your computer is first starting up. The key to use depends on what BIOS you have, but there should be a message at the bottom of the screen that says something like "Press F5 for setup". If you don't know anything about a BIOS the information will be gibberish (SO DON'T CHANGE ANYTHING) but look for things that are "obviously wrong", that is the bios reports that a device has failed, has an error, etc., and look to see if there are any settings for a VGA device and if so, can the BIOS find the device?
But, I think the link that Jake posted points to your ultimate solution, but I doubt the registry values that person suggested will work for you. So this is what I would try next:
Boot into safe-mode and check the Device Manager. You may have devices and drivers show up in safe mode that don't show up normally.
If that doesn't work defrag your harddrive, run scandisk and then try reinstalling Windows again. However, when you get to the point where Windows says that it has detected a previous installation select the option to "Repair the previous installation". And make sure you are installing Windows by booting from the Windows CD.
If that doesn't work... remove the graphics card. If you have a spare graphics card of any kind lying around put that in and reboot. If things look ok, shut down and replace with your original card, hopefully things will still be ok when you restart. If not it most likely means that your video card is dead.
If you don't have a spare graphics card just reboot without one. Your computer will attempt to start, you (should) hear three short beeps from the speaker inside the case then your computer will shut down. The three beeps is the BIOS's way of telling you it can't find a graphics card, but the "beep code" may vary depending on your BIOS. Put your graphics card back in and reboot into safe mode and check the Device Manager again.
HTH