If you've got a spare supercomputer that is just sitting around waiting to test pre-released software on, then you'll be glad to hear that Microsoft has made the Community Technical Preview (CTP) of Windows HPC Server 2008 R2 available on Microsoft Connect.
Windows HPC Server is a specialized version of Windows Server designed for use in high performance computing applications, designed to solve complex computation problems or in parallel (cluster) computing. Minimum system requirements for Windows HPC server is an x64 hardware platform such as Intel Xeon processors with Intel Extended Memory 64 Technology (EM64T) or AMD Opteron processors and 512MB of RAM. However, we suggest picking up a Cray CX1, starting price is around $25,000.
While Windows HPC Server is based on the same technologies that are in Windows Server 2008 R2, the HPC version isn't designed for use as a general purpose server such as a web server, file/print server, etc.
Microsoft boasts improvements that focus on scalability and manageability of HPC infrastructures in the R2 release, in addition to enhancements in the job scheduler and a more robust programming model and runtime support.