kypen already mentioned http://www.driverheaven.net/patje/
tool. Basically, it look like this.
1. Uninstall old drivers.
2. Run wxp-w2k-catalyst-8-09-041221m-020455c.exe and let it unpack files in default "C:\ati\..." derectory and then it will fail.
3. Run Dhmodtool2.1.exe, by default it will choose the same directory. You have to choose your card from drop-down list and press "Start mod" button.
4. Watch as it will modify couple files and offer you to try to install the drivers. Agree and restart system when it finish.