In WinProxy,
1.  Click on File => Settings => Protocols.
2.  Check the box next to 'Automatic network configuration (DHCP)'.
3.  Check the box next to 'DNS', then click on 'DNS Setup'.

4.  Under "DNS Server IP to add" enter only the primary and secondary DNS server addresses of your ISP.  (Remember to click 'Add' when entering each server address.)
5.  Next to 'Domain' enter in the domain name.  This should match what you entered for Domain under the DNS tab of the internal network card on the WinProxy computer.

6.  Click OK => OK.