OK. I installed MCE 2003 OEM into a Virtual PC 2004 window.
When the MCE install routine asked for "Windows XP Professional CD#2, I hit "Cancel".
The install of MCE (XP Pro) continued with no error messages. It accepted the OEM MCE 2003 Key that I had.
The "System Control Panel" reports that I have: "Microsoft Windows XP, Media Center Edition, Version 2002, Service Pack 2" installed.
After the install (never using CD #2), I tried to join it to an SBS 2003 Domain using the "ConnectComputer" Wizard furnished by SBS 2003.
When running the Connectcomputer wizard in IE, the Domain Join Failed:
"An error occured when configuring networking settings. See your network administrator."
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FinishNetworkingSetup() -- calling NetJoinDomain( NULL, RTAIT.LOCAL, ou=sbscomputer, ou=.........)
FinishNetworkingSetup() -- NetJoinDomain() failed [50], returning
FinishNetworkingSetup() failed -- hr == [-2147467259]
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So, this "easy" solution doesn't work. The standard answer for those wanting to convert XP MCE 2005 into XP Professional may be the best:
Reformat the hard drive, buy XP Professional, and install that.
Of course, there are still those hacks that people have done to allow MCE to join a domain. I haven't tried those yet. Also, I didn't allow the MCE install to join the domain during setup. Supposedly, this works. But the probem is that, if you ever have to leave the domain, you can't rejoin.