Hi,
please tell us exactly which Virus was found, and where (full path/folder/filename).
See avast's report/Log for this info
try booting in SafeMode (F8-Boot) and repairing the infected file there, or moving it to the chest..
P.S.: This sounds also as if your avast could be damaged; you might want to try repairing it via control panel -> Software/programs -> Uninstall avast -> repair