xp pro computer stopped going to internet

I have a LAN that uses an internal DNS/DHCP server. I have two computers that have stopped going to the internet. I've checked them using ipconfig /all and their settings look fine. No changes there. I can ping and see every other computer on the network. The DHCP server uses our Cisco router as the gateway and I can ping that. I cannot ping the firewall from those two computers. I can ping the firewall and go outside to the internet on every other computer. I've gone to device manager and checked the settings on those two computers and they seem fine. I have restarted my firewall and I have not made any changses to the firewall. We did have a power blip about a week ago and I had to restart the firewall, router, and internal switch/hubs.