What is your trojan scanner ?
More importantly, what is the malware name, the infected file name, where was it found e.g. (malware name, 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.
It may be that your trojan scanner unpacks its signatures to scan and these are detected, this is speculation on my part as i don't know what scanner you used or the file name of the detected file (above questions).
