So I installed a new ATI video card. But every time I boot up I get an error message telling me cannot find ASUS video card. The old card is gone. I've deleted the programs that came with it from Add/Remove Programs and uninstalled the drivers in the Device Manager. It's not listed under display properties. Yet I'm still getting the error message. Where else do I need to delete references to it to get the error message to stop?
First guess: try reseating the card and the RAM. Sometimes that is all that is wrong.
My second guess is that you need to go into the BIOS and either disable the integrated graphics (of the Mobo, usually abbreviated as IGP in the BIOS), or set the "order" of the video so that the BIOS checks the PCI first for video card, before defaulting to the IGP. My latest NVIDIA card required this, and the installation caused me to scratch my head til I realized the BIOS options were so cryptic that I hadn't done that.
(If that's not clear, search the net for your specific BIOS by version/maker and get the config options for video.). Note the BIOS is like god to the OS, so a setting in BIOS might actually deny the OS permissions and therefore result in an OS error message.
Alternatively, you may need to install the ATI drivers, then reboot the computer, but that is not likely I think. If you've tried updating the drivers already (i.e. loaded the drivers from the included disk, but also found newer ones online), it may also be that you need to do a "clean install" of the drivers.
Another possibility, was that the old video card wasn't the culprit, and actually part of the mobo was damaged somehow, so no video card in the particular slot will work (might try seating in another PCI express slot, if available). It's good to have a second CPU seated in another mobo as a test bench for components when stuff like this happens, though there are also "readout cards" that give info on the mobo/cpu when crap like this happens.