Thanks AO !
There is a website somewhere that carefully explained to me that Avast was off its rocker. (Huh! Can't now find that url).
I can't seem to find the website you refer to, however a quick Google suggests Avast has been issuing false positives against Adobe Acrobat on and off since Acrobat 7 !
You would have thought they could have figured out a fix by now, obviously some hard-coded stuff lurking around the depths of Avast code.
gemma10, forgot to mention it earlier, but I believe Adobe sign all their binaries these days, so you can just check the signature is kosher if you want to do something extra to put your mind at rest.