I haven't used it (yet!), but you can bet its on my list of things to play with.
blurb from their website:
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.

