the case like what happen to me..
my analysis. it's happen beacuse the virus infected the file who has ".exe" extension.
oneday, my laptop had a problem. there's a trojan win32:vitro infected almost all programs who have ".exe" extension.
example: winrar, winamp, all my game..
but there are still a file who can't infected by this virus. the file who strong enough.
example: office 2007(word, excel, etc), tuneuputilities, avast, and manymore (forget)
so, the virus infected the weak file who have ".exe" extension. and the avast detect it as virus.
then, it can't be used anymore. because no executeable program can run.