As you've probably noticed when starting the game, you get asked if you want to run the DOSBox version or the Windows 95/XP only version. If you pick the latter in Windows 7, you get to pick the executable (what Steam actually starts is a CD crack which asks for the game's original executable, Terror from the Deep.exe, which can be found in the \steamapps\common\X-COM Terror from the Deep\TFD folder). This will result in a black screen followed by the game crashing.

This is a quick guide on how to get it working. The process can be severely simplified if you already own the game 7th Legion on Steam, which already uses the patch I'll suggest (the GOG version works as well). If you own that, all you need to do is to go to that game's Steam install folder and copy the files agrit.cfg and ddraw.dll into the \steamapps\common\X-COM Terror from the Deep\TFD folder.

If you don't, the process is just a few steps longer:

• First, go to http://bitpatch.com/ddwrapper.html and download the ddwrapper.zip (direct link, but may be out of date at the time you read this)[bitpatch.com] file at the top of the page.
  
  
• Open the zip file and extract the files agrit.cfg and ddraw.dll into the \steamapps\common\X-COM Terror from the Deep\TFD folder (if you don't know where your Steam install is, right click the X-COM Terror From The Deep entry in your Game Library, select "Properties" in the menu, then click the "Local Files" tab and the button labeled "Browse Local Files").
  
  
• Next, open agrit.cfg in your plain text editor of choice (for example Windows' Notepad).
  
  
• Edit it (or copy and paste the below) so it looks like this:
  
  ;;; www.bitpatch.com ;;; RealDDrawPath = AUTO BltMirror = 0 BltNoTearing = 0 ColorFix = 1 DisableHighDpiScaling = 0 FakeVsync = 1 FakeVsyncInterval = 1 ForceBltNoTearing = 0 ForceDirectDrawEmulation = 1 NoVideoMemory = 0 SingleProcAffinity = 1 ShowFPS = 0
  
  
• Save and exit notepad, and try playing the game. Keep in mind that Steam's CD crack will ask for the Terror from the Deep.exe in the TFD subfolder every time.

Also remember that this patch will be removed if you ever verify the game install or reinstall the game.