Okay, most people don't update their PCs, or atleast down download the latest drivers. You should definately do both.
i remember on my laptop, i hadn't updated my PC. Could barely play Crysis on low. Now that i've updated it, i can run in Medium/High!
To update your drivers for nVidia PCs click
Here.
To update your ATI PC click
Here.
Update should be on by default so you shouldnt have to worry. Messages will pop up all the time. If not go to your control panel and find Windows Update in classic view on Vista, or the normal view on XP.