Another simple method for changing MAC address in windows xp

Goto Start->Setting->Control Panel, and double click on "Network Neighborhood".
       

1.       Select the Network Adapter you want to change the MAC address. 
2.      Right click on the selected Network Adapter and click "Disable." Verify the status column for this adapter changes to "Disabled". 
3.      Right click on the selected Network Adapter and click "Properties"there select "configure->Advanced->Locally Administrated MAC address there u can set ur new mac address 
4.      Save it. 
5.      Enable the network card.

by using ipconfig /all u can observe the change.

And in windows 7 it may or may not work i will post a perfect procedure for windows 7 also soon