Windows MCE is a "flavor" of XP Professional that is intended to be a entertainment center that co-exists with your TV, stereo, etc. By design, it can not join a domain (although that can be cured) so it can work with what they call Extender Devices that play music and movies from machines on you local area network.
You will have to buy a new copy of MCE if you intend to install it as you current license does not allow you to use the same key for a copy of MCE.
Spend the $$ and go legal.