Some Network Adapter drivers have this facility built-in. If a value is present, windows will use it as MAC address, if not, windows will use the hard coded manufacturer provided MAC address. When the Network Adapter Device is enabled, windows searches for the registry value 'NetworkAddress' in the key HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlClass.
This software just writes a value into the windows registry.
Technitium MAC Address Changer allows you to change (spoof) Media Access Control (MAC) Address of your Network Interface Card (NIC) instantly.