When I play this full screen I'm just given a black screen and no amount alt-tabbing, changing graphics cards, compatibility options, admin rights or DEP will fix it!

Can someone help?

It's worth trying to change the DOSBox settings to see if a different one will work.

1. Open the following file in your favourite text editor (notepad.exe if you don't have one):
C:\GOG Games\Ravenloft - Strahd's Possession\dosbox_strahd.conf

2. Look for this line:
output=surface

3. Change it to this:
output=overlay


The config file has the following instructions related to that setting near the top of the file:
# output: What video system to use for output.
# Possible values: surface, overlay, opengl, openglnb, ddraw.

Let us know how it works out.

Thanks. I'll give this a try.