if you downloaded and installed a new antivirus on top of your old one you're going to have problems...running two antivirus programs at once tends to make them miss things.
if you are just running the one, there's a possibility that the virus compromised the install so it'll miss the virus. take frank's advice and try and online scanner.
it's also possible that your original scan found a false positive, or a file it thought was a virus but actually wasn't. the online scan should help you figure that out.
so - run the online scan and let us know if it shows anything. if it does tell us the name of the virus please