Since you have an OEM license, you can also order a Downgrade Media Kit from Microsoft, which will include the install media for Server 2008 and the associated key for the downgrade. There is a shipping and handling fee, but is otherwise free:
http://www.microsoft.com/windowsserver2008/en/us/downgrade-TC.aspxhttp://www.microsoft.com/windowsserver2008/en/us/downgrade-rights.aspx
