Unplug the modem? Thats not going to fix his DHCP-assigned (and cached) dns addresses.
Probably what you will see if you do status on your network connection on your PC's, or an ipconfig /all, is DNS server addresses that are 192.168.0.1 (the address of your router). Which is how its supposed to look when dns relay is enabled.
When it is disabled though, this entry should be something on the internet. An
Should do it.
If it doesn't reboot them.
I'll try an ipconfig /release & /renew then reboot each XP machine. Previously I disabled the NIC and enabled it which I thought did the same thing. Stay tuned....