antivirus preventing windows from shutting down

I suggest using AVG which is free. I haven't really heard your AV provider. But as a solution, try closing your AV first before shut down and try if it helps.

Try changing to another AV product. There are several good free ones available. I believe that Avast, AVG and Avira are currently the top rated ones. I use Avast and I don't see any significant speed reduction (though all virus checkers inevitably reduce speed slightly under certain circumstances). When Windows shuts down it always has to do some housekeeping and so does any other application which is running. This can delay shutdown somewhat but certainly should not prevent Windows from shutting down.