John,
I can't really think of anything else right now.
If I was in your position, and had a) a spare hard disk, and b) a burning, stubborn, passionate desire to crack this, I would:
- pull out the current HDD, put in the spare, load a clean install of Windows, add SP3 then load Avast, see if it works, then keep adding apps and testing until it fails (if ever), not forgetting windows patches.
Otherwise I'd give up and go back to a previous release of Avast, or try an alternative.
SD