So you're trying to run a server off your cable modem or DSL connection?
Check this website out:
http://www.dslwebserver.com/
You need a static IP, and you have to make sure your ISP allows you to do this. (or if you can have them upgrade your package to allow you to run a server.)
You do need an OS. Without one, the system will sit on its bum doing nothing. You can get a copy of Windows Server from eBay, or you could do everything for free with Linux, Apache, MySQL, and PHP.
You don't need anything fancy unless you have a lot of server-intensive things you need to do. (Serving up HTML pages is simple, running PHP scripts like phpbb requires more power.) You could probably get away with using a $299 home PC.