Well firstly there shouldn't be an easy way to white list a file in avast, so can you be a little more specific ?
e.g. what type of scan was it that detected it, what were you doing, etc.
What is 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