For Windows XP and earlier:

 

How to find your IP and MAC address on MajaNet

For Windows Vista and Windows 7:

How to find you IP address (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.