Thank you Essexboy.
I actually kept on trying to fix the problem after I posted my query, and managed to get Avast working again by simply re-installing and rebooting although I had to do it twice before it was fixed.
Should I adopt the maxim that if it works, don't fix it? Or should I go ahead and run FRST with the fixlist you posted in case there are still some hidden problems? I have since done some cleanup of stuff I don't need any more and note that at least one of the files in the fixlist no longer exists.
Next time I will be more patient, as your fix is clearly more robust than mine!