How do I change the MAC address of my network card?

I have used these instructions in the past and they work great

There’s a C++ command-line utility called Macshift that allows Windows XP users to change their MAC address to any other valid address. I’ve written about how to use it and how to create shortcuts to change your MAC address on-the-fly. I’ll first explain how to use Macshift for any MAC change, then I’ll show you how to make Windows shortcuts using the command-line options. I’ve also made a small script to make it easier to use, but the script isn’t necessary. Macshift usage
Macshift is a command-only utility, so you need to learn the options to use it.

From Here

Method 1:

This is depending on the type of Network Interface Card (NIC) you have. If you have a card that doesn't support Clone MAC address, then you have to go to second method.

a) Go to Start->Settings->Control Panel and double click on Network and Dial-up Connections.

b) Right click on the NIC you want to change the MAC address and click on properties.

c) Under "General" tab, click on the "Configure" button

d) Click on "Advanced" tab

e) Under "Property section", you should see an item called "Network Address" or "Locally Administered Address", click on it.

f) On the right side, under "Value", type in the New MAC address you want to assign to your NIC. Usually this value is entered without the "-" between the MAC address numbers.

g) Goto command prompt and type in "ipconfig /all" or "net config rdr" to verify the changes. If the changes are not materialized, then use the second method.

h) If successful, reboot your system.

Method 2:

This should work on all Windows 2000/XP systems

a) Go to Start -> Run, type "regedt32" to start registry editor. Do not use "Regedit".

b) Go to "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\ Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318}". Double click on it to expand the tree. The subkeys are 4-digit numbers, which represent particular network adapters. You should see it starts with 0000, then 0001, 0002, 0003 and so on.

c) Find the interface you want by searching for the proper "DriverDesc" key.

d) Edit, or add, the string key "NetworkAddress" (has the data type "REG_SZ") to contain the new MAC address.

e) Disable then re-enable the network interface that you changed (or reboot the system).

Method 3:

Use the program Etherchange

Method 4: ( windows 9x)

Use the same method as Windows 2000/XP except for the registry key location is "HKEY_LOCAL_MACHINE\System\ CurrentControlSet\Services\Class\Net" and you must reboot your system.

Go into properties for Network Connection. Click "Configure" next to desired network adapter and on Advanced tab you should have one field for MAC address. Usually it is named "Address", "Locally Administered Address" or something similar. There you have text box in which you can type desired MAC.

Take care that you turn off original PC (or change it's MAC also).

You could use the Technitium MAC Address Changer