Just downloaded and program will not let me register. The button will not submit. Please let me know if anyone has had this problem. Thanks. 
Try offline register.
Fill the form here ( http://www.avast.com/registration-free-antivirus.php ), and enter the serial numbers you recieve in the email.
This happened to me, and off-line key was not accepted either. Had to go back to 4.8. You may want to save the setup log for submission if you have to go back, since there was a call for aid…
is it on update version or clean installation?
Is your system time and date correct?