Thank you for the reply. I've turned off automatically installing recommended updates. But I get the same error when I manually downloaded other drivers for my card from AMD's website.
I just bought a new hard drive and installed windows. Everything went fine with the installation. Windows downloaded an update for my Radeon HD 6870 automatically, and I restarted. After the restart, my welcome screen had horizontal black lines running across it (each line separate by maybe 3/4 of an inch). After logging, the screen went totally black after maybe 45 seconds, and I had to do a hard reset.
I started in safe mode and rolled back the driver update. When I restarted, everything was fine. I changed my update settings so that windows wouldn't download and install the driver automatically. I then manually downloaded the most recent AMD catalyst software suite and installed the latest driver for my card. I restarted and ran into the exact same problem: several horizontal lines running across my screen and a black screen shortly after logging in.
I'm not so sure this is just a buggy driver issue. A couple of notes: (i) when I roll back my drivers, the device manager doesn't seem to recognize my card; instead, it just "standard vga adapter" or something like that (when I update my drivers, device manager does show the card name... but of course everything else is screwed), (ii) the driver that windows installs automatically is different from the one I downloaded from the AMD website. The Windows download gives me Driver Version 8.850 while I downloaded version 15.20 on the website.
For what it's worth, I happened to have a Radeon HD 6850 lying around. I got it from a friend who upgraded about a year ago. I'm honestly not even sure if the card is any good. Anyway, I installed the card, but when I turned the computer on, my mobo beeped 5 times and I got no display. Fan on the card was definitely spinning. Google tells me that 5 beeps from my mobo indicates a video card issue, which of course makes sense. I reinstalled my 6870 and was able to boot up just fine with the standard vga drivers.
Note: The only other thing I've downloaded with the new hard drive is google chrome. So, we're dealing with what I hope is a very narrow universe of potential problems.
Any help would be greatly appreciated.