Just to chime in with my 2 cents, like WoodyZ said it is most likely an OEM OS issue. If that's the case then you cannot move that license. The OS license is tied to the hardware. You can give Microsoft a call but if it is an OEM license I highly doubt they will activate it. I guess it doesn't hurt to try though. I ran into this when I P2V'd some servers that were using an OEM Server OS.
Regards,
Patrick