Does Avast free antivirus works really?

Hello,
I have a 16 GB Transcend pen drive. When I copy some data into my pen drive from my campus, the computer of my campus always detects a virus named comres.dll with USB Disk Security and Kaspersky Antivirus. But I am using Avast Free Antivirus which had never complained to me about the virus!!!
Avast Antivirus software really works to detect virus?
