have the windows firewall running?
or just have a normal firewall running?
does your domain name resolve to your ip address?
also, if your on a router, is port 80 forwarded to your comp?
and does
http://localhost work? (meaning is you webserver running)
and one last thing.. does your ISP even allow inbound connections on port 80.. i know my previous ISP blocked a lot of ports.. including port 80..