This is long but maybe some ideas for you. I would definitely make usernames, etc unique. Verify that your network cards in both machines are compatible as far as speed, i.e. both 10mps or 100 or 10/100. If yo haven't already, doublecheck cables and connections.
Also, Win2k sometimes has a problem(known bug) syncing to the network. Try turning off both machines, and then boot them up with the win2k machine starting after the Xp machine. Then check the network.
Are using TCP/IP or Netbeui network protocol? If Netbeui, you don't need the IP addresses. If TCP/IP, you'll need to use static IP addresses as someone suggested earlier. I got bit several years ago and couldn't get a win98 and win2k machine to 'see' each other unless I had both protocols installed even though I shouldn't have had to have both.
If you're not using a router or Internet Connection SHaring which simulates a router, DHCP won't work(auto assigns IP addresses). Just go into TCP/IP settings and specify the 192.168.1.x addresses. You have to use static IP addresses if using just a hub or switch rather than a router with TCP/IP.
Next thing is that the switch itself could be the problem if it uses 'autosensing'. If it's just 2 machines, you might want to test with a crossover cable instead and see if you get a better sync.
(take a look at these links at Microsoft
and
If you need more in-depth help offline, let me know unless it's against forum rules. I just joined today so not sure.