Cannot ping XP from NT/2000 Network

I have an NT network with WIN 2K desktops. All desktops receive IP addresses from DHCP on my NT PDC. I have about 10 contractors who use their own PCs (Win 2000 and Win XP). They receive IP addresses from our DHCP server. I am able to ping the 2000 PCs from the network (server and workstations) but I cannot ping the XP PCs. I am, however, able to ping the server and other workstations from the XP PCs.

Any ideas why I cannot ping the XP PCs?