If avast didn't detect it what did ?
Then I assume that you have sent the sample to avast ?
If you are not getting a virus warning that you believe is a new, undetected virus then if you can zip and password protect ('virus', will do) the suspect file and send it to virus @ avast.com (no spaces), or send from the chest (after adding it to the User Files section of the chest).
Give a brief outline of the problem (possibly a link to this thread), the fact that you believe it to be a either a new, undetected virus and include the password in the body of the email. Some info on the avast version and VPS number (see about avast {right click avast icon}) will also help.
You could also check the offending/suspect file at:
VirusTotal - Multi engine on-line virus scannerOr
Jotti - Multi engine on-line virus scanner if any other scanners here detect it.