What actions have you taken to try and resolve the problem (there are several choices when detected) ?
Safest choice is Move to Chest.
What Operating System are you using ?
What is the infected file name, where was it found e.g. (C:\windows\system32\infected-file-name.xxx) ?
Check the avast! Log Viewer (right click the avast icon), Warning section, this contains information on all avast detections.