Mihai Nita

Evaluating Windows Server® 2008 software does not require product activation or entering a product key. Any edition of Windows Server 2008 may be installed without activation and evaluated for an initial 60 days.
If you need more time to evaluate Windows Server 2008, the 60 day evaluation period may be reset (or re-armed) three times, extending the original 60 day evaluation period by up to 180 days for a total possible evaluation time of 240 days. After this time, you will need to uninstall the software or upgrade to a fully-licensed version of Windows Server 2008.

Download Windows 2008 server R2
http://technet.microsoft.com/en-gb/evalcenter/dd459137.aspx