XP Pro Media Center Edition/Domain

I replaced four identical computers in our computer lab that were connected to our domain with newer machines. Recently I’ve been bringing the older machines back online due to growth of staff. Three of them reconnected fine. The fourth I have yet to be able to connect because the “domain” button in the system properties box is disabled (grayed out). I’m on the administrator’s account. The only difference between this machine and the other three is this machine is running XP Pro Media Center edition. The others..straight XP Pro. I did do some minor maintenance before reconnecting all of the machines like upgrade RAM, run system and spyware tools, etc. Anyone have any ideas why this is?