>I found out that to set priority to low I can use SetThreadPriority() API function with THREAD_PRIORITY_LOWEST constant but I can't find that constant value and can't find the .H file to see how it is declared.
It's defined in WINBASE.H as THREAD_BASE_PRIORITY_MIN, which is subsequently defined in WINNT.H:
#define THREAD_BASE_PRIORITY_MIN -2 // minimum thread base priority boost
Mike Stewart