I've just replaced my Radeon HD 5770 with an HD 7850. I uninstalled the drivers for the old GPU using the ATI Install Manager, restarted the computer, and then switched out the cards. After turning the computer back on with the new GPU, the computer performed the same as it did with the old GPU still in and no drivers installed -- low resolution display, but the PC speed was fine. I used the driver installation CD to install the new GPU's drivers and then restarted when prompted. However, when it brought me back to the login screen, I noticed that everything was unusubly sluggish: The mouse cursor moves 4 seconds after you move the mouse, and trying to type in my password was equally slow.
I tried shutting down the computer from the login screen, but it never completed shutting down after many minutes of waiting. So I shut off the computer manually, then restarted it. Windows took a very long time to start up, and then I had the same problem as before. I managed to type in my password, but I've been sitting in front of a blank screen for about 10+ minutes while it tries to load the desktop (I'm posting this from my netbook).
I've read that it's possible that some drivers or driver elements may not be fully removed using the standard measures and may cause problems like these when you install new drivers, but there's not much I can do about that at the moment since I can't use the computer.
If it makes any difference, the old GPU was Sapphire and this new one is HIS.
Also, I switched out my RAM at the same time that I switched the GPUs. I don't think the RAM is the problem, since the computer was running fine until I installed the new drivers, but I'll try putting the old RAM back in here in a moment. The RAM is all GSkill Ripjaw series, btw -- old RAM was 2x2 and the new ones are 4x4.
Thanks for the response. I think I've solved the issue! When I restarted the computer in Safe Mode, everything was running fine, so I realized right away that it was a driver issue.
If anyone reads this in the future, here's what I did:
Rolled back GPU drivers in Safe Mode (since there were no previous drivers, it reverted to my stock video adapter)
That made my computer usAble again, so I used the AMD Install Manager to uninstall everything AMD-related
It's probably not necessary (or advisable), but I used Driver Sweeper to locate and remove any leftover ATI drivers, just to be safe.
Then I restarted the computer and went to the page Yoji linked to, downloaded and installed the drivers, and everything worked wonderfully after that.