User rights assignments

We have a network of five computers - the gateway is a WinXP machine and it is fine. Two others are Win2000 and work well. There are two other WinXP machines that everyone can see, but not access. Those two machines can access everyone else and gon on the Internet, share printers etc. Both those machines have a problem with the user rights assignments in that the area that says allow this machine to be accessed by the network is blank. When we try to update the info and give permissions for access, a message comes back that the database cannot be saved. This is a curly one. Can anyone give some ideas?