If it happens again, make a note of the name of the trojan. Try moving it to the virus vault, for later transmission to AVG.
Then Google the name. There are likely to be a number of computer help sites that have posted procedure for dealing with it. Unless you're unlucky enough to be the first user affected!
The procedure is usually more complex than just having the AV software delete or heal it, as it can leave traces in a variety of places, and if the trojan then downloads something, it can be bundled with stuff to reactivate it after it's been deleted.
You don't happen to remember the name, do you? I've had an encounter with a couple of these recently.