If as Frank said you can find the file and it is detected as malware at virustotal also send a sample to avast virus@avast.com zipped and password protected with password in email body and false positive/undetected malware in the subject.
Or you can also add the file to the User Files (File, Add) section of the avast chest and send it from there (right click, email to Alwil Software).
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.