*** Improved by Wrobinson on 9/10/08 ***
You cannot backup Active Directory in offline mode per se, as long as a Windows Server 2003 or Windows 2000 Server OS is running with the exception of booting into Directory Service Restore Mode (DSRM) which is not recommended, except when performing an actual restore of AD.
When you backup the System State on a domain controller, Active Directory is backed up along with it. Of course, it is possible to backup the NTDS.DIT file and transaction logs offline but again, that would mean booting into DSRM or booting a domain controller to a parallel installation of the OS or a “live” OS on a bootable CD/DVD or flash drive, etc., then making a copy of the .DIT file and transaction logs.
Windows Server 2008 introduces the concept of restartable Directory Services, enabling Active Directory to be stopped and started without having to reboot the domain controller into DSRM. This is just one of the many new and great features of Windows Server 2008.
For additional guidance on backing up and restoring Active Directory, refer to the following TechNet article, Active Directory Backup and Restore.
*** End Update ***
use NTbackup utility
backup System State data
or use third party softwares