When multithreading a game what do i need to think about when picking the Libraries when developing for pc? I have been looking at process.h and was wondering if there are any big downsides (like speed or stability) whit this library and if i should look at something else?
The only decent C++ multithreading libraries are TBB and Microsoft's PPL. Picking anything else is begging for trouble.
Your question is a bit vague ("a game"... any game?) but I doubt you will have trouble with a "library" so much as your actual engine design. The harder/est part is to code your engine in such a way is that multi-threading is implemented correctly; picking a library (probably more appropriately, a language) that handles this well is almost trivial.