I don't doubt it is likely to be an FP but you should confirm.
Pause the web shield that will at least allow it to be downloaded, don't execute/run it.
Check the offending/suspect file at:
VirusTotal - Multi engine on-line virus scanner and
report the findings here. You can't do this with the file securely in the chest, you need to extract it to a temporary (not original) location first, see below.
Create a folder called Suspect in the C:\ drive, e.g. C:\Suspect. Now exclude that folder in the
Standard Shield, Customize, Advanced, Add, type (or copy and paste) C:\Suspect\* That will stop the standard shield scanning any file you put in that folder. You should now be able to export any file in the chest to this folder and upload it to VirusTotal without avast alerting.
If it is indeed a false positive, see
http://forum.avast.com/index.php?topic=34950.msg293451#msg293451, how to report it to avast! and what to do to exclude them until the problem is corrected.