You need to actually uninstall the AMD/ATI drivers, not just delete them from the C drive. You want to make the driver switch to the new card as "clean" as possible to avoid any conflicts.
If you absolutely cannot find the current AMD/ATI drivers in the Add/Remove Programs list, forget uninstalling the old drivers, shut down the PC, install the GTX 580, boot up. At boot up, it will say "found new hardware", ignore that, and proceed to install the new Nvidia drivers (double click on them). During the install process, there is an option to perform a Clean Install. Click on that and follow the prompts, then reboot again once the process is finished.