@Dub Trance
I know exactly how you feel, it was doing fine already with Avast 6 until when I updated it through the program itself, by the way, I'm on windows 7.
The first issue I encountered was the pop-up that keeps telling me "complete your upgrade to avast", everytime you startup, by clicking finish button, it did nothing, if you reboot again, it still keeps on showing. I also experience the freeze/lock-up issue you are experiencing.
The pop-up issue was resolved by uninstalling the avast and re installing it with a new copy from avast site. But the thing that made me which to MSE (Microsoft Sec. Essentials) was the freeze/lock-up I experiencing, my machine was down to its knees that I cannot perform any single task. I think it has something to do with Avast 7 and some windows 7 machines, probably not being compatible.
By the way, I tried to install Avast 7 on a Windows XP machine and it was all good, no pop-up and no freeze/lockup.
And regarding your question, if you want the version 6 avast. Look for it on oldapps ->
http://www.oldapps.com/avast_antivirus.phpLook for the one dated "29 November, 2011 (3 months ago)" I always use this site whenever I want older versions of a certain app.