Just Xing out of the of the popup doesn't actually do anything other than close the popup - there is no implied No when you do that.
It is just strange that you are still getting it and others in this topic aren't seeing it now.
I wonder if an avast Repair might help - Try a repair of avast:
XP - Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and select Repair, click next and follow.
Vista, win7 or later - Control Panel, Programs & Features, uninstall a program, select 'avast! Anti-Virus,' click the Uninstall/Change and select Repair, click next and follow.
You may need to reboot after the repair.