My DCs OS are 2003/2003R2 and FFL:2003,DFL:2003
Wanted to create a new child domain with Windows 2008 but got various error and after creating the below  key I have promoted that 2008 DC successfully.

Create the below reg key and after promoting the DC remove that.


Click Add Value on the Edit menu, and then add the following registry value:

Value name: NT4Emulator

Data type: REG_DWORD

Radix: Hex

Value data: 0x1

Quit Registry Editor.

See the below link; it's the same issue with 2008: