Just use SMAC or SMAC-CL to spoof the MAC address on Windows 2000, XP and 2003! Easy!
SMAC - Windows GUI based, easy to use.
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SMAC-CL - Command line based, just a console version of SMAC.
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I have been using SMAC since 1/2003, and it has been the best I have used. SMAC is very intuitive, and idiot-proof :)