- What OS are you using?
- What was the virus name, what was the filename, where was it found
example (C:\windows\system32\infected-filename.xxx)?
- What makes you 100% sure it is a false positive?
If you are unsure the best action is first do no harm (e.g. don't delete) move it to the chest and investigate as you are doing now. Check out this thread -
http://forum.avast.com/index.php?topic=14473.msg122170#msg122170 - it may help clear the confusion about the OK button (your file hasn't been deleted, just avast stopping the file being activated (locked, I believe temporarily) to prevent a virus being run. You may even want to comment on the thread as a user, perhaps a little confused by the OK button!
The do Nothing action is virtually hitting the OK button, but I would say if you hit the cross 'X' at the top right of the alert window, it too would do nothing, but you may get the alert again.
If you are getting a virus warning that you believe is a false positive, 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, the fact that you believe it to be a 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.
You could also check the offending/suspect file at:
Jotti - Multi engine on-line virus scanner if any other scanners here detect them it is less likely to be a false positive. Note: The file will have to be outside the chest to have it upload and be scanned by Jotti.