NT boot issues.

Our NT workstations all receive an IP address from a DHCP server. When we boot, we have to wait about 3-4 minutes after we see the login screen before we can actually log on (the services all have to be completely loaded). When we assign a static IP, we don’t have this problem; users are able to log in immediately when the see the ctrl-alt-del screen. Does anybody know what’s going on, and how we can solve the problem with DHCP?