nvidia drivers often leave some parts on your computer even when uninstalled.
What OS are you running?
This link will take you to guru3d and the detonator destroyer application.
Next, download/install latest direct X.
Re-download your ati drivers overwriting the old one (might have received a bad packet) and download the control panel.
With your old GForce card in, install a generic PCI display driver.
Shut-off computer and install your Radeon card.
Install ATI drivers and then install control panel.
Hopefully, that will do the trick.