Evaluating any version of Windows Server 2008 R2 software does not require entering a product key, however will require activation within 10 days. Failing to activate the evaluation will cause the licensing service to shut the machine down every hour. Once activated, the evaluation will run for 180 days. After this time, you will need to uninstall the evaluation software and reinstall a fully-licensed version of Windows Server 2008 R2.
The only catch is that this installation requires Hyper-V, you will need to have a base install of Windows Server 2008 (x64) or Windows Server 2008 R2, running Hyper-V. You can find more details about using Windows Server 2008 R2 VHD with Hyper-V here. But if you have Windows 7 you can run this also. Check this post how-to.
Also download Changes in Windows Server 2008 R2 guide and Introducing Windows Server 2008 R2 e-book from Microsoft to know what’s new in the latest version Windows Server.