right click avast tray icon…pause shields

however you should send the file to avast so that they can check the file and fix detection

You can upload files and report issues to avast lab here: http://www.avast.com/contact-form.php (change subject according to Your case)

you can use mail

send to virus@avast.com in a password protected zip file
mail subject: False Positive / undetected sample (select subject according to your case)
zip password: infected

or you can send files from avast chest
how to use the chest. http://www.avast.com/faq.php?article=AVKB21