First, Click start, then run and type "dxdiag"
(Alternatively, you can right click on my computer and choose properties, Hardware, Device manager, Display adapters and it will show you the card. You can also right click and go to properties of the card for more detailed info)
A window should open, let it load for a bit and then at the top click the display tab and it should tell you what card you got.
Go to your card manufacturer's website, based off of the card. (Nvidia or ATI)
They should offer drivers and instructions, just be sure to uninstall your old ones first.