That's just really, really odd.
Have you tried the omega drivers for your card? They're not supported, but they're basically pieced/hacked together by some very experienced hardware gurus out of the best of each driver release. Not sure where to find them these days.
That's a simple answer:http://www.omegadrivers.net/
Just pulled them yesterday, to tell you the truth. Wanted to exploit my X800 XT to its fullest, so off to overclocking land I go.
In all honesty though, while you really should uninstall your older drivers before installing the new ones, that's something I never do. Yes, it's technically wrong, but I've never had any issues yet.
The only thing I can suggest is to try to download older drivers. I'm just not sure where to get them from.