Look in Add or Remove Programs to see if there are any ATI drivers listed in there. If you find any, remove them. and reboot. I'm hoping you are still running on the VGA (native Windows) drivers...that's where we need to be.
What model number Dell do you have...I'd like to see if there are some specific conditions we need to comply with. Dell machines can be a bit proprietory... Let me know on the model number and don't install anything until I get back to you.
Before installing the new drivers, make sure to disable any anivirus applications or other "registry protection" programs you have installed. We need to be able to write freely to the registry.