Microsoft has released an evaluation of Windows Server 2008 R2 HPC Edition
The Windows Server 2008 R2 HPC Edition operating system is a version of the Windows Server 2008 R2 operating system designed for use in an HPC cluster. Differences between HPC Edition, Standard, and Enterprise can be reviewed at Windows Server 2008 R2: EditionsWindows Server 2008 R2 HPC Edition Evaluation
System Requirements
Supported Operating Systems: Windows Server 2008 R2
CPU: x64 architecture computer, except for Workstation nodes which may run x86-based or x64-based processors
RAM: 512 MB
Multiprocessor support: Windows Server 2008 R2 HPC Edition supports up to four processors per server.
Minimum disk space for setup: 50 GB
Network interface card: At least one network interface card (NIC) is required. Additional NICs may be required depending on the selected network topology