Repair is rarely a successful option as it on;y works for files that have been infected by a true Virus and would have to have been included in a VRDB Generation, which is totally improbably for a flash drive.
The failure of the GUI could have been as a result of any number of reasons, attack being possible since that was what avast was trying to prevent.
What is the malware name, the infected file name, where was it found e.g. (C:\windows\system32\infected-file-name.xxx) ?
Check the avast! Log Viewer (right click the avast 'a' icon), Warning section, this contains information on all avast detections.
