Not a trojan, are you using XP? if so I think its the infamous plug and play problem.
From regedit go to the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\DirectPlayNATHelp\DPNHUPnP subkey.
Click on edit at the top of the screen and select New, DWORD Value.
Type UPnPMode as the name and press Enter.
Double-click the new value, set it to 2, then click OK.
Restart and see if that fixes it.