The Active Directory is running slow.

Hi all.
I have this kind of problem.
I have an Active Directory that controls a small network.
the problem is that the clients logon very slowly.
It costs a 5 mins or something for clients to logon. We are in local network.
I made a sys defragment and clean the unusual files on the main machine but this doesn’t solve my problem.
Can Anyone help me with this situation ?
OS: 2000 pro