there might be a couple of causes
did you ask if your ISP blocks port 80 for home users?
some isp dont like home user runing a small server
did you create all nessary html php or whatever language your gonna use, files (i.e. the website, did you create it already?)
if its php, did you check that you have set up the required options in php.ini?
How many computer go trough your router? the router assigns a local ip address for each computer, sometime this information changes when you restart the router.
Go to your router configuration page (i.e. if its netgear, it should be routerlogin.com)
and make sure incoming connections are being forwarded to your computer, and not to your bro/kids computer)
open your network connectio status window by doubles clicking you connectiong in the taskbar near the clock, click on tab that says "Support" and write down the local ip, should be something like 192.16*.*.*
make sure the port is being forward to that ip
this is kinda hard to explain but if you have any more trouble i.M. me or add me to yahoo ;)