Computer security has long been one of the most important research areas in computer science. In recent years, the rapid growth in Internet based industry has raised the importance of computer security to an unprecedented level. However, at the same time, profit driven commercial software development always leaves security concerns behind the quick incorporation of new functionalities. Therefore, the need to improve the security of these products is very urgent now. Microsoft Windows 2000, as one of the most popular operating systems, also needs to be improved. Especially, because of the unavailability of the necessary documentation and source code, few third party research and development have been done for Windows 2000 operating system kernel. In this paper, we introduce WinLomac, a prototype security enhancement software for Windows 2000 operating system that enforces Low Water Mark integrity model based Mandatory Access Control in the kernel.