Goto Start->Run, type "Winipcfg"
Select your ethernet card and record the MAC address
Goto Start->Run, type "regedit" to bring up registry editor. Make sure you backup your registry in case you screw up. If you screw up important registry entries, you may damage your computer system. Again, MAKE SURE you have a good backup of your registry entry!
Locate "HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\Net". It should have subkeys like "0000", "0001", "0002" and so on.
Click on each subkey that starts with "0000" and determine your Ethernet card by looking at "DriverDesc" key. If it did not match, then goto "0001", "0002" and so on...
Once you got the right NIC card,
Click on dropdown menu Edit->New->String Value.
Name the new string "networkaddress" and click OK.
Double-click on the new "networkaddress" string and you will be prompted for "Value data"
Enter the new MAC address you want to assign. The MAC address you enter should be a 12 digit number with NO "-", i.e. "00C095ECB761"
Here are 2 ways to activate the new MAC address:
If you have regular internal Network Interface Card:
Reboot your system
If you have PCMCIA Card, you can do the following without rebooting (Thanks to Andrew McGlashan for his input):
Select card and release the DHCP settings
Open "PC Card (PCMCIA)" -in control panel or on the task bar if set
Stop the card
Eject the card
Re-insert the card
Select card and renew the DHCP settings
Verify the new MAC address with "WinIPCFG"
Resources:MAC Address Spoofing for Windows 2000 & XP/2003