Well a dell restore would be an act of last resort and not something that I would consider.
The only windows update that I know of which caused some issues with avast, and that was for the latest round of April patch Tuesday updates. It was an update of C++ 2008 Redistributable (which was subsequently pulled), that changed the kernel32 or some such, but that appeared to be only an issue with win2000 and avast users. So I don't think that would have been an issue with Vista SP2.
I have not been a system restore fan for a very long time as it is far from infallible and often you get unexpected issues. I now prefer to have my own Image Back-up software and run that weekly, If I have a problem I just restore the last weekly image (exact copy of a partition/drive). This saves going down the starting from scratch road.
Given you have essentially done as I suggested, running the uninstall utility for old versions of avast also, I'm at a loss as to what it could be and I have never used Vista.
In your first post, you say it fails after a reboot, does that produce any error message or an entry in the windows event viewer ?
There may also be some .hdmp files in the avast folder, you could try a search for *.mdmp, these might help developers but I'm not to familiar with checking them with a tool from MS.
http://social.msdn.microsoft.com/Forums/en-US/isvwindowserrors/thread/501f077a-9c0d-48af-a530-83e3de8d0972/