Don’t make the mistake I did with Team Foundation Server 2008 media on MSDN downloads.
The MSDN file en_visual_studio_team_system_2008_team_foundation_server_workgroup_x86_x64wow_dvd_X14-29253.iso is the workgroup edition, as the file name suggests. The problem is that you cannot upgraded it to a full edition. TFS documentation says that a trial or workgroup edition can be upgraded by entering a valid CD-key when in Add remove programs, maintenance mode; however this option is greyed out if you install from this media. I checked with Microsoft and there is no way round this for this ISO image.
So if you want to install or upgrade to TFS 2008 full edition make sure start with the right media, else you will downgrade your installation to a 5 user workgroup.