How to reset domain admin password on a Windows Server 2008

1. Boot onto DVD of Windows Server 2008
2. Choose “Repair your computer”
3. Launch cmd
4. Go to c:\windows\system32
5. Rename Utilman.exe to Utilman.exe.bak
6. Copy cmd.exe to Utilman.exe
7. Reboot on Windows
8. Do the keyboard shortcut Windows + U when on the logon screen
9. net user administrator Newpass123 inside the cmd
10. log on with the domain admin account and this new pass
11. change the password to remember it if needed
12. Reboot on the DVD to put back the original Utilman.exe

For more info take a look at this link