Change your priority to THREAD_PRIORITY_IDLE instead of the default which is normal. That'll lower the thread priority so it won't take your system with it. The command would be:
CWinThread * threadptr = AfxBeginThread(
pfnThreadProc, // thread function
pParam, // pointer to data to use
THREAD_PRIORITY_IDLE, // the idle priority, was THREAD_PRIORITY_NORMAL
0, // stack size, 0 means default size
0, // create and run immediately, don't suspend on creation
NULL // no security attributes
);