Microsoft has created the Windows 2000 Active Directory Migration Tool, which you can download at http://www.microsoft.com/windows2000/downloads/tools/admt/default.asp.
Here is a quote from the above URL:
"The Active Directory Migration Tool provides an easy, secure, and fast way to migrate to Windows 2000 Active Directory service. As a system administrator, you can use this tool to diagnose any possible problems before starting migration operations to Windows 2000 Server Active Directory. You can then use the task-based wizard to migrate users, groups, and computers; set correct file permissions; and migrate Microsoft Exchange Server mailboxes. The tool's reporting feature allows you to assess the impact of the migration, both before and after move operations.
In many cases, if there is a problem, you can use the rollback features to automatically restore previous structures. The tool also provides support for parallel domains, so you can maintain your existing Windows NT 4.0 domains while you deploy Windows 2000."