You have to go to the Start menu and click on Run, or hit WindowsKey + R. A dialog box should show up.
Type 'cmd' (or 'command' on windows versions older than xp), like this:
Hit OK and a new window will appear. In this window, you have to execute the following command: ipconfig /all
After you execute that line you'll end up with some text on the screen that might look like this:
If your local IP is different than the external IP, it means that you are behind a router and that you are sharing the external IP with at least one device (the router being one of them).