Hello and Welcome to LinuxQuestions,
I Googled for 'Integrating Linux in Active Directory' and in 0,43 seconds got over a million hits. This is the first one
Linux, AD, and Windows 2003, and a pretty good one to explain what you need to perform this task.
With LDAP you can 'port' authentication to a Windows 2003 domain without a problem. I have it setup here for numerous webapplications running on Debian and all users authenticate through LDAP on the Active Directory domain. Works like a charm.
Kind regards,
Eric