Fixed it (UT2K4) for me mate - did you uninstall with Driver Cleaner?
Driver cleaner does not work 100% of the time. On more than one occasion I've found stuff left over that Driver Cleaner missed. No matter how many times I've run it, in normal or safe mode it doesn't always remove everything.
Running driver cleaner and installing the new drivers didn't change anything for me. UT2004 performance is still shit.