I just recently acquired Deadly Shadows from GOG (I own the Steam version, but I prefer GOG, no DRM baby, WHOOO!), ahem, anyway, when it comes to running the game under windows 8.1, there is only one problem I am having (I was having another issue where I would right-click on a chest to lock pick it on the second mission, but the sneaky upgrade, I believe that's what it's called, fixed that). My problem is the brightness slider doesn't function. I installed the game on a windows 7 notebook I have in my possession, and the slider worked just fine, so I believe it's a windows 8.1 issue. I haven't tried running the game in compatibility mode, but wanted to post here first to see if anyone had any suggestions (other than re-install 7. Windows 8.1 is pretty good!)

Before y'all ask:
Specs:
Intel core 2 DUO E6550 @2.3 GHZ
MSI P6N SLI Platinum
8 GB DDR 2 memory
Sapphire 6850 graphics card
Windows 8.1 pro X64

Please see the discussion starting here, especially posts #260 and #262. Also, make sure you have the newest SU (1.1.3.4).