NEVER install more than ONE antivirus EVER. Even if you disable all the automatic protection features on the additional antivirus(es), they will still, in all likelihood, conflict, and as a result cause errors on your system. These errors can very well damage your system to the point of having to reinstall it. Additionally, having more than one antivirus will lessen the protective ability of all of them substantially because of errors.
Here is a list of free registry cleaners, online virus scanners and tips on AV's in general that I posted: