Check out (and contribute) to the http://www.reactos.org/wiki/index.php/Untested_%28open_source%29_softwar... @ ROS wiki !
Portable ReactOS would be great, anyway...
For those who don't know ROS:
The ReactOS® project is dedicated to making Free Software available to everyone by providing a ground-up implementation of a Microsoft Windows® XP compatible operating system. ReactOS aims to achieve complete binary compatibility with both applications and device drivers meant for NT and XP operating systems, by using a similar architecture and providing a complete and equivalent public interface.