Amazon Elastic Compute Cloud (EC2) is the first general computing grid based on Xen. Amazon is testing the service since this summer, but at the moment new Xen capability to run Windows guest OSes is still unavailable.
To workaround this limitation Enomaly, the consulting company offering a multi-plaftorm management solution called Enomalism Virtualized Management Console (VMC), managed to run Windows inside EC2 with help of QEMU.
They prepared an extensive how-to for installing Fedora Core 6, QEMU and Microsoft Windows Server 2003 R2 on top. Read it here.