Computer numerical control (CNC) system is the base of modern digital and intelligent manufacturing technology. And opened its architecture and constituted based on PC and Windows operating system (OS) is the main trend of CNC system. However, even if the highest system priority is used in user mode, real-time capability of Windows (2000, NT, XP) for applications is not guaranteed. By using a device driver, which is running in kernel mode, the real time performance of Windows can be enhanced greatly. The acknowledgment performance of Windows to peripheral interrupts was evaluated. Harmonized with an intelligent real-time serial communication bus (RTSB), strict real-time performance can be achieved in Windows platform. An opened architecture software CNC system which is hardware independence is proposed based on PC and RTSB. A numerical control real time kernel (NCRTK), which is implemented as a device driver on Windows, is used to perform the NC tasks. Tasks are divided into real-time and non real-time. Real-time task is running in kernel mode and non real-time task is running in user mode. Data are exchanged between kernel and user mode by DMA and Windows Messages.