Ooh boy. You are quite ambitious, aren't you? It took Linus Torvalds several years to get a proper working version of Linux. An OS does so much you can't even imagine. It needs to interact with your hardware (usually through drivers), and at least parts of it must be written in assembly to interact with the low-level APIs of hardware components. A .exe boot loader as you call it can't be executed by the BIOS; .exe is a DOS / Windows executable so it needs to be started from an already loaded OS.