Microsoft Operating System Licensing under VMware
For those you with Microsoft Windows 2003 R2 licenses and some sort of virtualization (Xenserver or VMware), you might want to double-check to make sure you are not over allocating your licenses. Under Microsoft’s R2 licensing, 1 purchased copy of Windows 2003 R2 Enterprise entitles you to run 4 virtual machines. If you choose to virtualize the 38 Windows 2003 Enterprise servers and upgrade to the R2 licensing, you could potentially be licensed for an additional 115 virtual servers.
More information on Microsoft’s R2 licensing can be found here: http://www.microsoft.com/licensing/highlights/virtualization/faq.mspx