tcp/ip problem

I have a small network of win2k pc’s. I just made a ghost of 2 to make room for 2 new ones. I have tcp/ip and netbuis protocols on all. i have dhcp assigning all ip addresses. 2 people on the network say they can’t log on when everyone else is logged on. If tey get there early they are fine to log in. How do I correct this problem and avoid it in the future?