List of computers in my network places

We have a server Windows NT 4.0 with service pack 6a as a domain and there are cleints with Windows NT 4.0 and Windows 2000 prof. with service pack 4. We are also hooking the LAPTOPS with WINDOWS XP prof. and service pack 1a. Our IP Addresses are on DHCP.
Now, my problem is, when I am looking at my network places from laptops, system is not showing all the computers which are on network. It is showing only the clients with WIN NT 4.0.
When I search the client with win 2000 it will find out the computer and show it to me.
We have activated the TCP/IP protocol and NetBEUI protocol too.
What could be the reason? Please help me in this regards.