Would you care to expand, how (or what) was it detected ?
What was the file name, where was it found
example (C:\windows\system32\infected-file-name.xxx) ?
Don't delete it before sending the sample to avast, you could also manually add it to the avast chest. and send it from there.
If you are not getting a virus warning that you believe is an 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.