I'm running SBS 2011 with 15 XP Pro workstations. Everything has been running fine for about 6 months but a few days ago one computer had the yellow ! and would not connect to the network. I configured a static IP and worked fine. The next day three more PCs had the same problem and the next day all of the PCs were not being assigned IP addresses. I rebooted the server, DHCP services are running, firewall is disabled, etc. I've set all of the PCs up with a static until I find a solution to the problem.