Hi,
I am not sure but the port that you found open compare it with know backdoor ports I think there is a RFC on it, But because you said that you scanned for virus and nothing turned up i dont think there is a problem with trojans. but to be sure just get a good trojan removal tool,
abd if you dont have one even get a firewall it will help you keep the number of programs going online under controll. well my last gues could be that if you have compressed any drive this could be its host drive.