Avast! Antivirus it totally irritating me.

The worldwide accepted antivirus, Avast may be a boon for many of its happy users but I am way too much irritated with it since a few days ago. I had inserted one of my friends USB Pen drive in my PC and since then it always says “Malicious file found” even if the computer is ideal. Why does it happen? And if I leave my pc scanning the antivirus is not able to locate any malicious files. How to fix this?
