Did you ever get bored with your old MAC address? If you did, this is
the solution! Mac MakeUp let=3Fs you change the MAC address of any of the
interfaces present on your Windows 2000/XP/2003 box.

Sometimes this is referred as MAC address spoofing.
You can choose a new address of your choice, or get the new one with the
help of the tool, which sets the OID part according to your preference.
http://www.gorlani.com/publicprj/mac.../macmakeup.asp