Your sure what is not a virus ?
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. C:\Program Files\Alwil Software\Avast4\ashLogV.exe
- Or check the source file using notepad C:\Program Files\Alwil Software\Avast4\DATA\log\Warning.log and copy and paste the entry.