Before you go trying this, try DOSbox first. I worked all this out before DOSbox had compatibility with Apoc, but they now say they run it just fine, and it's far easier and more reliable than setting up all this stuff.
Install Xcom Apocalypse anywhere. The installer does actually work under WinXP, though it doesn't seem to like slashes, so you have to install it directly to C:. I then copied my installation directory to somewhere friendlier like C:/games/XcomApoc, and it still works fine. Do not set up a sound card yet - won't work.
Download and install VDMS from here: http://sourceforge.net/projects/vdmsound/. This is a library that emulates sound cards. The sound still isn't wonderful, but it's good enough to play the game.
Download the utilities mousectl.com, fakecd.exe and xcom3_cd.com. I'm not entirely sure you need all three, but that's what I have, and it works. Because it took me ages to find them, I put them and the batch file I use all in a blob here. Just unzip them all directly to the Apoc directory.
In the file "Run Xcom Apoc.bat", you may want to tweak the numbers after mousectl.com slightly according to taste - they control how fast the mouse moves. They seem to depend a lot on CPU speed or something. Always make sure there are exactly eight numbers - if you only give seven, it will work, but then use a default for the last one, which is far too big, and your mouse will jump all over the place.
Now run setup.exe and set up a sound card. Use "Soundblaster Pro", 220, 7, 1 for your settings (those are the defaults for VDMS, so obviously if you changed them, use the new ones instead), but do NOT try to test the sound card - VDMS is not actually running right now, so you'll probably just crash the machine. Save and Exit.
Right-click on XCOMAPOC.EXE, select "Properties", go to the "Compatibility" tab, and tick all the boxes - run in Win95 compatibility mode; Run in 256 colours; Run in 640x480 screen res; Disable visual themes; Turn off advanced text services.
I didn't do this in the end, but you may need to open up properties for XCOMAPOC.EXE, go to the "Program" tab, click "Advanced..." and use the AUTOEXEC.NT and CONFIG.NT files that are in the .zip above. Seems to work fine without them for me though.
You probably also want to disable your screensaver, or at least put it on a long timer. Xcom doesn't recover well after the screensaver kicks in. In theory you can go to the "Misc" tab and untick "Allow screen saver" - but that doesn't seem to help for me - screensaver still kicks in.
And then when you want to run Apoc, right-click on "Run Xcom Apoc.bat" and select "Run with VDMS". Apparently you can add some VDMS calls to the batch file and then you don't have to do this, but I didn't manage to get that to work.
And enjoy Apoc all over again. The Gollops don't seem to like how Apoc turned out, and they went back to turn-based stuff for Nemesis, and I agree that Apoc's turn-based mode is flawed (which is why I never play it). However, the pausable-real-time mode is excellent fun, and games are done a lot quicker than XCom 1 (Ufo Defence/Enemy Unknown). They're just different games, and I love them both.
Let me know if this didn't work for you, and what tweaks you had to make to get it to work, and I'll update this page. Spread the joy around and stuff.
Tom dot Forsyth at eelpi dot gotdns dot com
There's no place like home