Cooperative Linux (coLinux) is the first free and open source method for running Linux on Windows natively, without using commercial virtualization software like VMware. It allows users to run Linux alongside another operating system on a single machine, such as Windows 2000/XP/Vista/7, in a more optimal way. The unique aspect of coLinux is that it uses special driver software on the host operating system to execute the coLinux kernel in a privileged mode, giving it full control of the physical machine’s MMU. This allows coLinux to achieve almost the same performance and functionality as a standalone Linux. coLinux also shares hardware with the host OS through emulated devices and can be easily ported to run under other operating systems.
http://www.colinux.org/