Most people have a random IP assigned to them by the ISP each time they connect to the internet. These IPs are taken from a large block the ISP owns.
If you were to use the DOS command tracert IPAddress you could probably get a pretty good idea of where they are located though just by reading the router names.
Or VisualRoute is good too. It can be found at http://visualroute.visualware.com (free registration required).