Free software that acts as a separate Linux Operating System on top of Microsoft Windows. It is a Linux system suitable for beginners that allows users to run Linux and thousands of programs for the OS without complicated dual-booting or format/installing. CoLinux is also suitable to run vast numbers of Linux "machines" in parallel without much overhead.
CoLinux takes users a step closer than the UNIX emulation software like Cygwin. The first release has already been shown to run a Debian system. It is the "first method for optimally running Linux on Windows (and other operating systems in the future) natively. It is a port of the Linux kernel and support code that allows it to run cooperatively without emulation along with another operating system."