Brief Yipee! moment when I updated Avast and it let the game run. Then it didn't. Poodoo.
I've downloaded the game from the site, rather than running it through Steam, and I'm still getting errors either on stage 3 or 5 of initiation - either an exception breakpoint (which I never saw on the Steam edition), or the normal GC DuplicateHandle failure. Adding the game or the game folder to the exclusion list in avast doesn't seem to be doing anything.