Did you receive any message during your attempts to install? It would be really informative to post any messages that are blocking your update here.
You can, as the installer suggests, disable your Antivirus and try to install the update. I've just installed it here without issues, but I'm on Avast (free) antivirus. I've fulminated on the generally poor behavior of AVG in the past, and won't repeat that here, but my best suggestion would be to switch to a different antivirus solution.
