Thanks, all! Now I have several things to try if it gives me any more trouble, including repair and possibly a complete cleanout and reinstall, but as I have it nicely configured right now and it seems to be working I'm going to give it a couple of days and see if the virus def files are now being managed correctly. Seems a shame to do a complete reinstall if it was just a minor glitch.
I should mention that when I said that I got some error messages about files not found after I restarted AvastSvc.exe, the files in question were the old virus folders which were now in the process of being deleted, not program files or anything like that. At the time I was running Process Explorer to try to track down file handles and had been trying to delete stuff with Windows Explorer so who knows what was going on!!