An option: remove the driver and install it again. But - as you don't tell why you can't update - there is a risk that it won't install the way it did the first time (if, for example, the CD is damaged). So be sure your System Restore is functioning.
But be warned, it's strange that your asus en8500gt shows as 'standard vga graphics adapter'. It should show as an asus en8500gt. That might be the explanation: something is wrong with the card. If Windows doesn't recognize it, there's a good chance that the driver won't recognize it either, so won't install. Could that be the cause you forgot to mention.
If you delete it then, you might need your Windows Vista DVD to reread the standard VGA-driver that came with Vista.
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