Johny - due to the way home routers work etc, your router is being assigned your external ip address and your computer is being assigned an internal one.
If you want to access your web server as if you were outside of your home network (for example, anyone else trying to view your server) you will need to go through a proxy - since I'm in the UK and on blueyonder broadband I can use webcache.blueyonder.co.uk, try and find out what your ISP provides in the way of a proxy. If they don't provide one, check out
http://www.stayinvisible.com for a list of available proxies.
As Segosa said, you will also have to "forward" all requests on port 80 from your router to your computer. To do this, login to your routers web interface, and choose port forwarding / virtual server / nat (or something similar). You will then have to forward requests on Port 80 TCP and UDP to your machines internal IP address.
Save your settings and you should be ok now. It is best to setup your computer to use a static IP address, rather than a dynamic one, so that you don't need to change the routers config if the IP address changes.