Users of Avast Antivirus will be finding many of their programs will be falsely showing as infected with a trojan. This includes other well known programs like Skype, Spybot S&D, Nexus and Copernic for example. This is obviously a false detection (known as a false positive) and at the moment Avast's forums are swamped with people letting them know about it.
Avast have now updated their definitions file correcting this problem, so update those first before trying anything else. To update the Avast database manually right-click on the Avast icon in your system tray and select "Updating" from the menu and then select "iAVS Update". Click it and it will update to the latest version.