A number of possibilities.
Let's try the easy one first. You obviously have an nVidea graphics card. Look to see if your card has an updated driver.click here and/or
Vista usually offers updates to video drivers though updates. If you go to Control Panel then select check for updates. If nothing, in the left pane choose change settings and make sure that recommended updates and update service are ticked. If they were not then do the check for updates again.
Post back here if this does not help, there are other things to check.