Well, the OS is multithreaded, or you wouldn't be able to run but one program at a time.
Many programs are multithreaded, not just games. They use timeslicing, each thread within the system gets to execute for a given time when it needs execution, then focus moves to another thread. There is where priorities come in. NT systems have 6 priorities, normal, above and below normal, low or idle, high, and realtime. Realtime is exclusive operation. The reason the computer appears to freeze in realtime operation is because one or more threads are set to exclusive operation, which may rely on the OS to draw graphics or perform RPC calls, but can't execute, so it will wait indefintely (freeze) for something that will never happen.
Multithreading has been around for a LONG time, not just in *nix and Windows. It exists in the first versions of DOS to a degree.