It depends on wht your goal is. If you want to do is restore the DC due to a
hardware or software failure not because you want to recover deleted objects
then restore your backup including the systemstate. See this KB:
240363 HOW TO: Use the Backup Program to Back Up and Restore the System
State
http://support.microsoft.com/?id=240363
If you are trying to recover deleted objects then you will need to do a
restore of the systemstate and mark the object that you are trying to
recover as authorative. See:
241594 HOW TO: Perform an Authoritative Restore to a Domain Controller in
http://support.microsoft.com/?id=241594
If you are recovering deleted groups then also see:
280079 Authoritative Restore of Groups Can Result in Inconsistent Membership
http://support.microsoft.com/?id=280079