General Notes on Steam Distribution of XCom Series Games
Take Two's 2K division is the current owner of all the XCom series of games.
2K have made the games available via Valve's Steam service, at http://store.steampowered.com . (The exact link is currently on the main page of this UFOPaedia.)
The cost is $4.99 per game, or the entire collection of 5 games for $14.99.
In the Steam distribution, the XCom executables and game files are found in e.g. "C:\Program Files\Steam\steamapps\common\x-com terror from the deep\TFD" and "C:\Program Files\Steam\steamapps\common\xcom ufo defense\XCOM".
XCom 1 (UFO Defense / Enemy Unknown) is the DOS 1.4 version, from strategycore.com.
XCom 2: Terror From The Deep (TFTD) is also a DOS version, possibly the version sometimes called "TFTD Beta".
Steam provides anti-cheat functionality to prevent modification of the games distributed via Steam. However this fortunately does not seem to be fully implemented for the XCom series. The game executables are patched to force them to chain to the Steam launcher first, and the Steam launcher chains to a file configured by Steam (in runme.dat). However the target file can be changed to point to a different target, or the target file can be overwritten (e.g.by the contents of runxcom.bat).
The game data files themselves can be edited using standard tools such as XCOM Hack / TFTD Hack, and the command line version of XComUtil. The data file changes are then correctly picked up by the game.
XCom 1 works fine. Just edit dosbox.conf autoexec, after you install using xcusetup. The only issue may be problems seeing messages or prompts from runxcom.bat if you opt to use the inter-executable actions such as xcomutil after-combat messages, or autocombat.
TFTD is now pretty much as described for XCOM 1 (see below for full details).
Copy xcomutil files into the "XCOM" subdirectory of the "xcom ufo defense" installation directory. Open a command window in the XCOM subdirectory and run XCUSETUP. It should work normally.
Then edit the autoexec section of dosbox.conf, replacing:
Launching XCom 1 from Steam should now provide all the normal RunXCom features.
There may be issues if runxcom needs user input during the black screen between Battlescape and Geoscape. You will probably be unable to see any messages or prompts. Just hit enter. If that doesn't work, hit ctrl-C. If you want to see what the prompt was, check xcomutil.log.
TFTD is now also distributed on Steam as a DOSBox version, possibly the version identified as "TFTD Beta" which apparently includes some research tree bug fixes. Again, after copying all xcomutil files to the "TFD" (sic) sub-directory and running XCUSETUP, edit the autoexec section of dosbox.conf and replace "call terror.bat" with "call runxcom.bat".