There is another possibility. A lot of times, ISPs will block incoming requests to certain ports. So it's possible that your ISP is blocking port 80 (HTTP).
What you can try is in IIS you can configure another port. In IIS manager, go to your default website, rightclick->Properties. Go to the "web site" tab, click on "Advanced" button. You can then click "Add", leave IP Address as "All Unassigned", enter a TCP port (usually people will use 8080), then "OK" until you are back to the manager.
Now from the other PC, you enter http://youripaddress:8080/
and see what happens! If that doesn't work, try another port, the ISP may block that too, but most likely not. I have to use port 81 on my cable modem. Usually dial-up providers wouldn't block port 80 cause most dial-up users aren't likely to run a web server (but there are exceptions!)
As far as the subnet mask, you can ignore that. That's just another thing a that TCP/IP needs to work.