Chris-
From KB Article #325379
Verify that domainprep completed successfully. To do so, verify the
following items:
a.. The adprep /domainprep command completed without error.
b.. The CN=Windows2003Update,CN=DomainUpdates,CN=System,DC=dn path of
domain you are upgrading exists
You didn't mention which version(s) of software we are talking. But this
info refers to 2003 (preparing to go to 2003 from 2000).
BS