Thanks for the reply. Could it be a specifically Win7 issue?
Its still happening almost a week on but I've just run the AVAST repair option and maybe that will fix it.
The definitions updates must be being installed considerably slower than normal as I tried not logging on for 30 seconds and still had the pop-up. However if I leave it a couple of minutes it doesn't appear.
I thought about going to Manual updating instead but surprise, surprise AVAST doesn't want you to do that and adds a warning triangle to the tray icon even if the definitions are up to date. It says my system is not secure just because I've turned off automatic definition updates - that is stupid.