If you have Windows Server running on your LAN, you can configure Active Directory, which supports LDAP right out of the box. See How to Configure Address Book to Query Users in Active Directory for details on how to configure Outlook; follow the instructions in the Downlevel Client Considerations with the Address Book section to set the server, user, password, and port in Outlook's LDAP service.
In Outlook 2003 and later, LDAP is installed as part of the address book service. To enable an LDAP directory, go through Tools, E-mail Accounts to add the LDAP address book to your profile.
If directory browsing is enabled on the server, you can browse the address list. Otherwise you will need to use search to find people.
See Configure LDAP options in Outlook 2007 on TechNet for more information and the registry keys used to control browsing.
LDAP provider that supports browsing of the address book (which Microsoft's LDAP doesn't do) with additional features and an ActiveX developer's kit available
In Outlook 2003 and 2007, browsing (when supported by the server) is controlled by a registry key.