Open up the start menu, and click Run. You should now see the following window.
Type command in the Open: box, and click Okay. If command didn't work try cmd. This will bring up a black command prompt window.
The command prompt may look different on your screen, but it doesn't really matter. Type ipconfig /all in that window, and then press the enter key. This will dISPlay a lot of information. If it scrolls off the top you may need to enlarge the window.
In the list of information that went scrolling by look for the Physical Address. The physical address is the MAC Address of this computer.