According to this document from Microsoft (http://www.google.com/url?sa=t&rct=j...WatFNyBU_6DhNg
As long as you have licensed Windows 7 Enterprise edition AND have a CURRENT software assurance agreement on that software, you can install and run 4 additional copies of the OS. These do NOT need to be Windows 7 Enterprise edition, they can be any prior version of Windows...including Windows Vista Business and Windows XP PRO
In the event that your Software Assurance contract expires you can continue to run the Windows 7 Enterprise copy on the physical machine...but the Virtualization Use Right to run 4 other VM's does not
Unfortunately, all of the above is not really applicable to retail customers....as this option is only available to volume license customers with active software assurance contracts. As a stand alone end user, you cannot obtain or license a copy of Windows 7 Enterprise edition for home use.