Hi,
please report the exact message of avast, including trojanname & full path/folder/filename
ad-Aware sure does have a temp folder; but it's probably only a temporary dir..
it's maybe used by ad-aware for unpacking or downloading new detections..
if in doubt, on next occurrence -> let avast shield move the file to another folder, and scan it with onlinescanners Trend & KAV (see below)
send it to virus@avast.com if it's a false alarm..