MS Hyper-V lets you run GNU/Linux as a guest

Windows Server 2008 Hyper-V, the next-generation hypervisor-based server MS virtualization technology, allows you to make the best use of your server hardware investments by consolidating multiple server roles as separate virtual machines (VMs) running on a single physical machine. With Hyper-V you can also efficiently run multiple different operating systems—Windows, Linux, and others—in parallel, on a single server, and fully leverage the power of x64 computing.