How to Join a WMC to a Domain 

If you have tried joining a Windows MCE PC to a domain you quickly found out you couldn't do it unless you did it during the actual OS load process. There has been a pretty lengthy "hack" for fixing this called the 'Banana Hack' which does work but as I said it's pretty lengthy. Here is a quicker 10 step process that has worked for me just fine.
  1. Boot into Windows using a WinPE CD/DVD
  2. Start Registry Editor, click on HKEY_Local_Machine
  3. Click on File, Load Hive
  4. Select c:\Windows\System32\Config\System file
  5. Give it a name: MCE
  6. Browse to HKEY_Local_Machine\MCE\WPA\MedCtrUpg
  7. Change IsLegacyMCE to 1 (ONE)
  8. Browse to HKEY_Local_Machine\MCE
  9. Click File, Unload Hive
  10. Reboot the PC
On reboot you should be able to go through the normal Join Domain process however if you are on SBS and use the Connect Computer function you will probably find the system joins the domain but the client information is not migrated from the local account to the new domain account so you will have to do a manual migrate of that information.