There were major security changes between 2003 and 2008 that could create some mild difficulties with clustering. If this is for demonstration purposes, you may want to use all three versions. However, a real world scenario can even involve a Windows NT server combined with XP client systems, due to lazy management. If you really want to show the full spectrum, you could use Windows Server 2003, 2008, and 2012. The obvious pick for the most stable environment with the current Windows 7/Windows 8 set-ups and AD/group policy compatibility with Windows 7 and Windows 8 would be Server 2008 R2 and Server 2012. It really depends on what the role of your servers will be. If its a simple domain controller and you want to demonstrate basic group...