Hey there. I have a copy of Windows XP MCE 2005, but it looks like the install disk got lost when I moved. I still have the key, because the COA is stuck to the side of my computer.
Basically, because XP MCE was never made available as a retail product, you'd have to request replacement media from the system builder. If the system builder has gone out of business there are alternative procedures available to get a replacement from Microsoft, directly as an end-user. You do have to provide details of the company you purchased the media center PC from though.
Failing all of that, best to find somebody who has a MCE install disk and make a copy of the thing. The disks aren't copy-protected.