IIS - Other - The page cannot be displayed - Server Not Found or DNS Error

I have a XP Pro machine (SP3) running IIS 5.1 at home.
I have exposed the HTTP port 80 on my router (D-Link DIR-655) to go to this machine.
From my web host, I'm redirecting a subdomain to my home PC's external IP address.
From my laptop (at home), typing the subdomain on IE/FF it redirects me to my home ip address and the website loads.
From another laptop (also at home), the same happens.

When I do this outside of my home network, even though I'm typing the same external ip address, I get the "The Page Cannot Be Displayed" error.

I've put the machine hosting the site on the DMZ and allowed the router to reply to pings.
I can ping the IP address remotely with success, so I know the IP address is good, but for some reason, one when I'm within the network, does the website load.

Any thoughts?!

So IE 6 says "The page cannot be displayed" and at the bottom it says "Server Not Found or DNS Error"
Firefox says "Network Timeout" and "The server at xx.xx.xx.xx is taking too long to respond"
I've tried again about 20 times and always gets a timeout error.

I've tried disabling the firewall, enabling the firewall with port 80 for TCP and UDP open, and to no avail.