Bob, I think if the ONLY thing you've done is update Avast after a fresh (and normal) reboot, then the variables are quite limited.
There is certainly a problem for SOME Win7 users.
If you then remove it and the problem goes away, it becomes as close to definite as you will get I would think.
A fresh install and the problem is solved.
Not quite sure which other factors could be at play, but given we are sorted now, it possibly doesn't matter to me (except that the other computers in the house will get a "clean" install rather than an update).