You could try a few things, if you haven't already:
1) open device manager, locate your video card there and delete it. Then try running the setup that came with the ati card.
2) likewise in device manager, locate your video card and just disable the driver, or click the update driver, show all hardware, have disk, and point it to the .inf file that came with the ati card.
Naturally enough, I have never seen an error like this before, so all I can do is give you some general pointers.
You could also get a bit more hard core with the system:
Windows sees a piece of hardware by seeing the vendor and device ID's that are present inside your ATI card. It then looks through your .inf files, which are located in c:\windows\inf
All the .inf files are simply text files. One of those text files contains the same vendor and device ID values as your ATI card, so that's the one windows reads up. In that .inf file, it tells windows what driver (a .sys file and corresponding .dll/other files) to load to get the hardware to work.
Soooo, you could paw around in those .inf files, looking for vendor and device ID's of:
VEN_1002&DEV_5A61
and simply rename any/all of them you find to something other than .inf
Reboot windows and it'll be very, very confused and should then ASK for drivers to load.
The vendor ID of 1002 is ATI.
the Device ID of 5A61 is something I pulled up off google. It may or may not be exactly your card. You could examine the .inf file that came with your driver set for that card to get the exact number you need, then go searching from there.
Yes, this is fairly dangerous work and you can wreck a system doing it, but I somehow think you aren't very far along in the O/S installation anyway, so if you have to nuke it and start over, no harm done.
good luck,
-jeff!