Many thanks for your reply to my problem.
I use Avast as my antivirus (free version) As far as I can tell, it doesn't have a section for allowing 'safe' programs.
I can take screen shots just fine on other programs - it's just Myst which is causing the problem - it's Myst, End of Ages - the latest game.(a few years old now)
The only other protection I have is Windows Defender.
I write sci-fi - if you would like a free e-book I will gladly send you one (as attachment - which will open in any word processor), just email me firstname.lastname@example.org
If you have any other suggestions, I would be grateful.
Using printscreen, I can only take one shot at a time - and then have to close down the game to access the 'shot'. I need to take multiple shots to string them together in a panorama.
Kind regards, Ravin Dave