Having poked around in the forums I have found that no one seems to believe, at least not at the time of this writing, that Windows Server 2008 R2 can be deployed on Xen or, if it can, that you cannot do so using the Xen package available in Red Hat Linux 5. I am proud to announce that Windows Server 2008 R2 does indeed run just fine on RHEL 5.4. I believe that the update to the latest 5.4 package is likely required here and, as Server 2008 R2 is 64bit only, you have no choice but to be running on x64 hardware. But work it does and 2K8 R2 on Xen on Red Hat Linux is very much a reality.