Cannot completely uninstall program

My computer came with a well-known anti virus program, which I uninstalled in order to use a different one. Now (years later), I am trying to install a different program that is saying I need to uninstall the original one that I had already uninstalled. I went to the program listing, and there it was still listed, but showing no disk space used. I cannot get rid of that listing in the program list, so cannot install the new program. Can someone help me with this? I am pretty much a computer idiot, so please be kind and very specific with instructions.