If you really want C (not C ++), I suggest using standard POSIX threads on platforms other than Windows and using pthreads-win32 on Windows. It supports both 32-bit and 64-bit, both MSVC and MinGW. The current version (2.9.1) was released just a month ago, so the project is actively supported. There is also a fork on github with some fixes in the MSVC2010 project.
If C ++ is also an option, I would choose boost, because where standard C ++ evolves (design std::threadin C ++ 11 is evolution from boost::threadetc.)
boost::asio - , ++ , C. , boost::asio I/O Completion Ports (IOCP) Windows, . boost::asio , , , ( ) .