You need to install the virtual network device called virtnet. Then you can change its MAC address to whatever you want. You can find the virtnet driver here http://www.ntkernel.com/?Products:Network_and_Security:VirtNet. Install it on windows 7 by going device manager -> action -> add legecy hardware -> next -> install the hardware that I manually select -> show all devices -> have disk (and here you select the netVirtNet.inf file) -> next -> next -> finish. Then you can change the MAC adress by going to the new devices ppropeties in device manager.