What was the filename, where was it found
example (C:\windows\system32\infected-filename.xxx)?
Have you tried a google search for the virus or infected file name ?
If you are 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.
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 or false positive 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.