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 'a' icon), Warning section, this contains information on all avast detections.
The reason I ask is that Panda's on-line scanner has a habit of dumping its signatures on the system and they aren't encrypted, so avast might be detecting those.