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Here we ******* go again.
I've jumped from nvidia to ATI and back countless times...never had issue with either. All I ever did was uninstall from the control panel, reboot and install new drivers.

Well i hate to be one of the few but yes i did have the problem the op is having after i switched from my 7800gtx to my X2. I did the whole control panel and driver sweeper thing but it missed some obscure file sorry cannot remember what it was called but it wasn't anywhere i would have expected to find it or thought it would cause a problem. I did a full clean install of vista (yeah i know pain in the backside) and touch wood it has been plain sailing since. So if nothing else do a clean install and that should sort it for you i have to say personally apart from that one hiccup which was my fault as i was being lazy and didn't want to do a fresh install i have had no problems with the Ati drivers.
Another case of "user error" with ATI drivers eh?
The only suggested solution I've seen people post is a fresh install of Windows, please explain how installing the drivers on a fresh install as opposed to installing them on an old install is a reflection on the users ability? it sounds more like a buggy installation procedure to me.
If ATI owners put as much effort into helping the OP try to fix it (without drastic measures such as a format) instead of putting all their effort into defending the drivers tooth and nail there might be less posts like this in the future.
Rule #1 in software design:
The weakest component is always the human. 95% of problems are due to failures by the biological unit as my lecturer used to say.
