@Tom_Wood: it depends on what kind of license you are trying to install. An OEM license, enterprise license, or retail licenses will all work. MSDN also works.
Dan Stolts "ITProGuru"
Dan Stolts "ITProGuru" is a technology expert who is a master of systems management and security. He is Chief Technology Strategist for Microsoft, owns several businesses and is a published author. Reach him on his primary blog http://itproguru.com or twitter @ITProGuru. He is proficient in many datacenter technologies (Windows Server, System Center, Virtualization, Cloud, Etc) and holds many certifications including MCT, MCITP, MCSE, TS, etc. Dan is currently specializing in system management, virtualization and cloud technologies. Dan is and has been a very active member of the user group community. Dan is an enthusiastic advocate of technology and is passionate about helping others. See more at: http://itproguru.com/about