Here's someones solution that worked for me:
I was having the same issue, just rebuilt an older machine (Athlon XP 2600+ with 1.5gig of ram and a Radeon 9550) to try Windows 7.
After a few hurdles which I semi expected on relatively old hardware, like the USB mouse not working when I plugged a jump drive in, then having to get the driver for the onboard LAN port, etc, I was faced with trying to find a way of getting the driver for the 9550 installed.
The long and the short is: download the XP version of the driver, then unpack it and let it give you the error saying that you need to be running Windows 2K/XP and stop.
Then go into the folder it unpacked to (default is c:\ATI\Support\9_3_xp32_dd\ though the last folder will be different if you use the 64 bit version) and right click the Setup application, go down the menu and select properties.
Then click the Compatibility tab and check the first box to run it in compatibility mode as Windows XP (Service Pack 3), click Apply and OK.
Then you can double click Setup, it'll run through the install and restart the machine, that should be you sorted with the driver installed, if not then I apologize because it worked for me!