For Windows XP and earlier:
For Windows Vista and Windows 7:
The universal way:
1. Press the Winows button on the keyboard (the window on the button), and without releasing the first press the button of the Latin (English) letter R
2. In the window that appears, write - cmd
3. Press Enter or the OK button
4. In the appeared black window, write - ipconfig / all
5. We are looking for a line - IPv4 * - that's your IP.
* There may be several of them, but most likely you will have one.