login to PC only if network cable is unplugged

I have several users that are unable to log into their PC if the network cable is plugged in. If they take the cable out then they can log into the PC. After they are in, they plug the cable back in and they can access everything they need. We have added the PCs back to the domain and they work correctly. A few days later, they call back with the same problem. Any ideas? These are all Dell Latitude laptops with either Windows 2000 or XP.