Open the avast virus chest - right click the avast icon, select Start avast! Antivirus, Menu, Virus Chest, then click on the User Files icon, File, Add, then from the navigation window select the file. Once in the chest, right click on the file and select email to ALWIL software. Enter some information on the problem, that you think it is an undetected virus and send.
Or 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).
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.
Sending from the chest seems less complex.