^^ He is asking about Operating System not windows and for creating Operating System you should have a good knowledge of various complex areas of computer science from hardware to software like OS algorithms, Process Management, Semaphores and you should have a good grasp from Low level Language Assembly Language to High level languages like C, C++, Python, Shell Scripting, Frameworks like Django. Moreover OS is a ongoing process which evolves over the time so you should be good at bug fixing, patching with the version control system like CVS, SVN, Git. It will be better if you take a piece of code in the OS and start contributing to it then you will have a knowledge how the process is maintained. One good project I can tell you is that http://www.linuxfromscratch.org/ ie Linux from scratch project. You can also contact me I am also working on something similar to that. Remember : There is not a single piece magic code that creates OS but its the handshaking between different codes and different languages and different process which creates OS. It will be better if you start looking the concepts of OS, Computer Architecture in depth and a start having a good handle on languages like C, Python and Shell Scripting.