I would say definitely uninstall the old ATI drivers before installing new Nvidia drivers. It's surprising you cannot access the uninstaller from the control panel (maybe, somehow, they are already uninstalled?). There are a couple/three things you can do:
1) Install CCleaner and use the "Uninstall" function under "Tools"http://www.filehippo.com/download_ccleaner
2) You can uninstall drivers from the Device Manager:
My Computer>Properties>Hardware tab>Device Manager>Expand the Display Adapter line>'right click' on the ATI graphics>select Uninstall and follow the prompts
*If you do not see an ATI graphics card listed, then there are no ATI drivers loaded.
3) Just install the latest drivers from Nvidia, currently at WHQL version 260.99, when prompted, select "Clean Install".