Ok. I'll break it down nice and simple to make it smooth for ya.
First, go to the website for yuor video card. Nvidia.com or ATI.com more than likely. Download the newest driver for your card and put it on your desktop and save it for later in the process.
Second, go to start>>settings>>control panel>>System>>Device Manager. On the left side of the device manager, find display adapters and expand that menu. Right click on your video card there and pick properties. When that window appears, choose the driver tab. Next, choose to uninstall or remove the driver.
After you do that, the PC will ask you to reboot. Go ahead and do that. When the PC comes back on, you'll get the found new hardware wizard. CANCEL that because you have the new driver on your desktop and don't want windows booting an old driver you may or may not have in the OS.
So just cancel that wizard. Now, just double click the driver file on your desktop. That should load the new driver nice and easy for you. Now, sometimes, if you have a badly corrupted driver, you may need to load the new driver in safe mode. If this is the case, just follow all the steps as above, but boot into safe mode before loading the driver in the last step. That will ensure you get a clean install.
Hope this helps.