IIS is on Windows XP, but where is your DNS server? And if you have no domain registered, nobody outside your network will be able to access your server by name anyway.
There is no Microsoft DNS for Windows XP You can install a third party DNS server, or use Hosts files, but frankly, you're not ready for either of those options. Do yourself a favor and register a domain, find a web host and put your site there instead of on your local system. Use the host's DNS.
Jeff