I had upgraded my Avast to Avast 4.8. It shows an on-access message saying there’s an infected file in blah blah blah but didn’t take any action. I had to scan again manually to re-detect the virus before clean it. Why is this happening?
Can you say what is the infected file name, where was it found (C:\windows\system32\infected-file-name.xxx)?
What avast! version and virus database are you using? (see About dialog of avast!)
Let your USB drive plugged and run Autorun Eater or Flash Disinfector, allowing them to clean up all drives. They would create hidden folders named autorun.inf in each partition and every USB drive plugged in when you ran it. These folders protect your drives from future infection. After that, reboot your computer.