I had to reset the domain administrator password on Windows Server 2008 because the password was lost (not by me for a change!). I ran across this very nice youtube video by Mathieu (Chateau?) that worked perfectly: http://www.youtube.com/watch?v=Ar-VoO9ogHc.

I'm kinda paranoid about useful webpages being removed, so I am writing down a description of the video here just in case:

• Insert the original Windows Server 2008 installation DVD.
• Reset the computer, boot from DVD.
• Choose option "Repair your computer".
• Start command prompt.
• C:\
• CD C:\Windows\System32
• MOVE Utilman.exe Utilman.exe.bak
• COPY Cmd.exe Utilman.exe
• Restart windows
• At the login screen, type Windows-U, this fires up cmd.exe now
• NET USER administrator <new password>
• Presto! Login as administrator!
• Now repair computer again and move the original Utilman.exe back.

There are a bunch of commercial tools to reset the administrator password, but there is no need for it! Furthermore most of them seem to reset just the local administrator account's password, rather than the domain administrator password.