The reason for having the same accounts/passwords on both machines is so you don't have to put in authentication information each time you access a network share. Unless the share has the access rights to all, and security on the file system is set to all, you will need to input a password each time you access. Place the same username/password on the machine with the share gets around this.
If you are in a domain, DC or AD setup the machine with the share checks the permission levels of the user accessing the network resources. Your setup compares security based on it's local information. IE it's own local users/passwords.
You have to have, in a non-domain setup, a local account or you cannot login to the shared resources. Hence the basis of people recommending the same user/password on each side.
One thing to check would be to make a share on your XP machines, and access those through the Win2k machines. I had a friend who could not access shares on a Win2k machine from WinXP due to the XP firewall.