That's odd because you should get a yellow question mark and then it should install the drivers for this unknown device.
Try this: Go into safe mode, uninstall everything ATI, run driver cleaner, then.... navigate to the C:\Windows directory, delete the directories "LastGood" and "LastGood.tmp", and rename "Driver Cache" to something like "Driver Cache OLD"
Reboot and then see if that doesn't help. If it does, go back and restore the name of the driver cache directory and enjoy
