It can be just left there for now. What AVG does with a virus it doesn't have a cure for is to isolate it in a safe place where it won't do any harm (the Virus Vault). It is likely that a cure will be available in the future and when it is, the infection can be 'healed'. Are your virus definitions right up to date? If not, download them and see if AVG can deal with it then.
Did you also have the option to just delete it?
A better explanation of AVG functions can be found
here. This does refer to AVG Professional but it applies to the free version as well.