Please help..I am having trouble uninstalling Autocad 2007

Has anyone out there had trouble with Autocad 2007? I have a copy (given to me by someone - not purchased) and recently tried to upload it. My computer keeps telling me that I already have it installed but I don't. I had Autocad 2004 previously and I uninstalled it prior to installing Autocad 2007. I tried giving my computer a registry shower but it keeps giving me the same error message. If anyone has any suggestions, I would much appreciate the help. Thanks.