IF standard installation does not work and you are under Windows 7, you can try this (also manual installation : get the OpenXcom zip archive)
1) Create folder OpenXcom
2) Create two subfolders OpenXcom\Data and OpenXcom\User
3) From Steam installation folder, copy following folders: GEODATA, GEOGRAPH, MAPS, ROUTES, SOUND, TERRAIN, UFOGRAPH, UFOINTRO, UNITS and paste them under OpenXcom\Data
4) Download Universal Patch, extract it and paste files into OpenXcom\Data (also should merge some folders and asking for replacing files : answer Yes)
5) Extract OpenXcom archive and paste files into OpenXcom (also should merge some folders and asking for replacing files : answer Yes)
File structure should be as above (folders between brackets) :
[OpenXcom]
[Data]
[GEODATA]
[GEOGRAPH]
[Language]
[MAPS]
[Resources]
[ROUTES]
[Ruleset]
[Shaders]
[SoldierName]
[SOUND]
[TERRAIN]
[UFOGRAPH]
[UFOINTRO]
[UNITS]
Openxcom.png
README.txt
[User]
- xxx.txt
- xxx.dll
- Openxcom.exe
Eventually zip it now for any further purpose, like reinstalling it due to mods that would make it crash
Then install some mods (also under Data) if you want, and activate them after start up (under Options > Mods)
And start game with OpenXcom.exe...