For ReportNet, nothing needs to be done to the AD environment. ReportNet, if using the AD provider, will connect using the Kerberos protocol, same as when you log into Windows. For the BI and Planning products, the dsheuristics setting must be set to 0000002. This is basically to allow anonymous connections ... it makes 2003 the SAME as AD 2000 was as far as anonymous connections.
Nothing needs to be done in 2000 to make the BI products work. If you can't add users, you may want to see that the baseDN is NOT at the root of the domain.
If you're domain is dc=domain,dc=com, then make sure that your baseDN in Configuration Manager is NOT set to dc=domain,dc=com. Try using o=Cognos,dc=domain,dc=com. The reason for this is that there is a Users folder that is at the root of the domain in AD by default. Cognos tries to create a users folder (different object type than the default Users folder) under the baseDN. So if it points to the baseDN, the users folder does not get created that Cognos needs ... hence the users will not be able to be created.