Support for parallel programming is very essential for the efficient utilization of modern multiprocessor systems. This paper focuses on the implementation of multithreaded runtime libraries used for the fine-grain parallelization of applications on the Windows 2000 operating system. We have implemented and introduce two runtime libraries. The first one is based on standard Windows user-level fibers, while the second is based on nanothreads. Both follow the Nanothreads Programming Model. A systematic evaluation comparing both implementations has also been conducted in three levels: the user-level thread packages, the runtime libraries and the applications level. The results demonstrate that nanothreads outperform the Windows fibers. The performance gains of the thread creation and context switching mechanisms are reflected on both runtime libraries. Experiments with fine-grain applications demonstrate up to 40% higher speedup in the case of nanothreads compared to that of fibers.