Look in the lower right corner of your screen and see if you don't have an icon for Avast. All the antivirus programs I've ever seen have an icon there that you can right-click and get an option to temporarily disable checking.
I am using Avast 7 Pro. From time to time when I wish to install a software program, I am told to turn off my AntiVirus program. Is there any way short of uninstalling Avast and then reinstalling it to turn it off while installing software?