Slow LogOn Process in Windows 2000 Server

HI, im writing from Venezuela and i hope that you can help me?
Many users have complained, when they logon on the Windows 2000 Server domain, takes to much time to end the autentication process, all computer Client are Windows XP Professional and the Windows 2000 Server have the Service Pack 4.
I have done many thing to solve the problem like:
– i have checked that the hard disk have a lot of free space
– i have checked that the server doesnt have viruses
– and another more..

What can i DO?
Sorry about my English…:(

Hans Meyer