I'm testing my application for compatibility with Windows 7 64 bit. I'd like to do this in a clean virtual machine. I know that Microsoft Virtual PC only supports 32-bit operating systems. Will Microsoft Virtual Server allow me to create a Windows 7 64-bit client?
No, Microsoft Virtual Server 2005 R2 only supports 32-bit operating systems and software. (See the Virtual Server 2005 FAQ page at http://www.microsoft.com/windowsserversystem/virtualserver/evaluation/virtualizationfaq.mspx.)
If you have Windows Server 2008 or later, Hyper-V will allow you to create 64-bit virtual machines. (Of course, this requires 64-bit hardware with hardware-assisted virtualization and a 64-bit edition of Windows Server 2008 as the host OS, since the Hyper-V role only exists in the 64-bit editions. See the Virtualization with Hyper-V FAQ at http://www.microsoft.com/windowsserver2008/en/us/hyperv-faq.aspx.)