XP Pro login to Win 2000 Server

I have put together a network with 5 XP Pro client computers logging onto a Windows 2000 Server computer. I created individual accounts for each person in the “Computers and Users” section of the Active Directory. Also, the computer names are in the Computers section of the Active Directory. Even though I have the individual accounts in the Users part of the Directory, not everyone is able to log onto the network using their individual accounts. Everyone is able to log on using the Administrators account. Can anyone please help me trouble shoot these problems?