Can a key-less install be done with Windows 7 Pro from Digital River and enter later the product key for the Full version?

"Yes, you can do a Custom Install and enter the key later. You have 30 days to activate an installation of Windows 7.

The first system you active an OEM version on the only system the key is legally allowed to be reactivated on. So if you activate it now, and then try to activate it again on a different system later, you are breaking the terms of the EULA."