1. Without information no one can say ?
How do you know there is a virus in your USB, is it dormant (not active or attempting to run when the USB is connected, have you scanned the USB drive, etc. etc.
2. Google has a historic database and avast's scans are real time. Avast isn't actually saying it is clean, just that it hasn't found anything on the pages you have visited. You also don't give any information on what google finds, etc. So the google issue is something you have to take up with the site owner and google.
This site also finds nothing and it is real time checking, http://sitecheck.sucuri.net/scanner/