What operating system (OS) is needed for trainer / mod / cheat to work ?

All of our trainers should work on all operating systems, as long as game version is correct (trainers only work on specific version of game).

We usually test trainers only on Win 7 64-bit, however if you have 32-bit, it makes no difference at all. It will work 100%. Vista and XP are also no problem, trainer will work.
We do have machines running XP and some trainers are tested on XP too.

Problem with most games, is that they won't run on XP anymore, so vista or 7 is a must have.

Linux - No idea, but if you can run a game on linux using Wine windows emulator, i'm quite sure trainer will work too.

Unix - bah .. never tried, but again if you can run windows applications, such as games, im sure trainer will work just fine.

Windows 8 - we have not tested anything on windows 8, but some people have had some problems that trainer interface won't show up, this is most likely because they do not have .NET framework or because win 8 is originally made for touchscreens and don't even have start menu by default, so there might be issues with showing interfaces.
We recommend not using windows 8, it is not meant for gaming, but there sholdn't be any issues running a trainer if you have all latest .NET frameworks and DirectX and other things needed for gaming.

Mods

Mods will work on any operating system. Mod (modification) is term used when original game files are altered, so there won't be any 3rd party application (such as trainer).