"Access" the server? Like for file sharing? Logins? The web? FTP access? Remote access? VPN?
Is the Windows 2000 server set up properly for Macintosh access? Are the Macs set up correctly to access the Windows 2000 server?
Please elaborate on your problem.. "Some Mac can't access the server" is extremely vague, and, as such, can only garner equally vague answers.