If you're looking for your own IP address, you should know that it is probably dynamic - meaning that every time you connect to your ISP, you get whatever address happens to be available from a pool. Even with broadband, it changes from time to time.
If you're looking for IP addresses of websites visited from your computer, check your web browser's history (although there are ways around this). Your ISP probably also has a log - which would most likely require a warrant for the authorities to obtain.
Sorry to hear about your identity theft problems... hope everything works out.
