I had a problem with my graphics card. My computer hung and I rebooted and the screen went into VGA mode. I tried bringing up Catalyst Control Center for my ATI card, but it refused to start - no GPU found! So I thought something's gone wrong with the card, and went to Fry's to get a new one.
Fry's didn't have any of the graphics cards I wanted - they had a couple of really high-end $500 cards, slightly discounted because they had been returned, and then a whole bunch of $50 graphics cards, not as nice as the one I had. I finally got a cheap nVidia one. I had a similar problem a few months ago when I went to Fry's to get a new monitor. I wanted a high-res one. They had exactly one - 1920 x 1200 ASUS so that's what I bought. Used to seem like Fry's had everything, but not so much these days. On the other hand, I got a couple of 8gig USB flash memory cards there for $6 each, a bargain.
I removed the ATI drivers, removed the ATI card and installed the nVidia card and its drivers. Same problem! Booted up into VGA mode, though it could handle 1920 x 1200, but graphics GPU not found.
After going down many false paths, I finally figured out the problem: I needed to install all the latest Windows updates. Not at all sure what's going on here, but I'm happy that it's working again. Now the next mystery: the Windows Experience Index (right click on My Computer and choose properties to see this) is lower for the new card. I would go back to my old card, but the computer feels faster with the new one! Again, a head scratcher but hey it's working again and I have more important things to worry about.