Portable devices are showing as affected or threats found

Hello,
My pc is recently going through a troublesome issue. I use avast free anti-virus latest upgrade and it is serving fine. Recently all of the portable devices I plug in are shown as affected or there is this message says threats found. No matter how many times I scan it there is no use. Is that a software problem or a hardware problem. I need to know for sure others wise I do not which way to go.
Please someone inform me as quickly as possible.
Alway White
