DHCP

I have a Windows NT 4.0 server and have DHCP setup on it. I have a few computers that when they log on they get an error staying that there passwords are incorrect or access to the domain server has been denied. I know that their passwords are right and they have access rights. What else could be the problem? Also when I look at the event viewer there is a warning about the IP address assigned to them a NACK error. I?ve tried to release and renew the IP I even went, as far as to give them there own IP. However when I do I get a BAD_ ADDRESS in the DHCP database and they still cannot log on. Other computers can logon just fine.