are you only having problems access the computer from the internet?
HIGHLY likely your ISP has port 80 blocked.
In IIS set the default port to 8080 and then try a URL like
http://68.100.100.100:8080
Also if you try it from the internet make sure you're using your routable IP.... NOT your internal IP (meaning DO NOT use the 192.168.x.x IP as it is useless on the internet)