For those of us dinosaurs still running XP and who used the windows XP workaround to play X-Com:Enemy Unknown, Enemy Within can be made to work by following the same steps as detailed in this post with a few addendums:http://forums.steampowered.com/forums/showthread.php?t=2964408
In order to get Enemy Within to work in the same way, make a second copy of "zernel32.dll" and "fileextd.dll" (which you downloaded to get enemy unknown working) and place them in the following folder.
Then, use the hexeditor which you used to edit "kernel32.dll" into "zernel32.dll" the same way you did for enemy unknown except this time modify the file "XComEW.exe", which can be found in the same folder as above, again replacing all instances of "kernel32.dll" with "zernel32.dll".
Note: One instance of "kernel32.dll" in the hexcode that you will find is capitalized, I don't actually code so I don't know if there's any significance to that, but I found that when you come to that one simply replacing the capital K with a capital Z will work fine. It may be unecessary for all I know, but I didn't bother to find out as doing this will work.