It would be best to download the newest graphics drivers directly from AMD/ATI here: Graphics Drivers & Software
When uninstalling the Nvidia drivers, you needed to make sure you were only uninstalling the graphics drivers. There should have been an option to remove only the graphics drives and leave the chipset/motherboard drivers alone.
If you haven't done so already, you should reinstall the motherboard/chipset drivers. Doing so will not affect the ATI drivers.