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.
@BrianKnoblauch, Blame yourself for not having a tool to identify networks with identical MAC addresses. – Pacerier – 2015-04-06T16:00:51.987
8Ugh. I hate it when people do that. I've been the unlucky person that ended up troublshooting network failures due to multiple identical MAC addresses on the same broadcast domain. MAC address cloning is evil, please don't do that unless people are dying all around you you have absolutely no other option to save them. :-) – Brian Knoblauch – 2009-10-02T12:57:58.473
4@Brian, you downvoted b/c his Admin is on vacation? – hyperslug – 2009-10-02T13:07:21.057
2What do you advise Brian? – Artur Carvalho – 2009-10-02T17:42:55.200
1Clone the old PC's but add 1. or 2. or whatever. MACs are pretty random, it's not likely you'll have 2 sequential on the same network. – quack quixote – 2009-10-02T17:55:45.597
4@Artur, you could also swap the MAC addresses of two PC's to prevent collision, if someone inadvertently plugged the old one back in. – hyperslug – 2009-10-02T21:49:59.977
@hyperslug good suggestion – quack quixote – 2009-10-04T02:06:40.200
1one example where hyperslug's suggestion will fail (causing the problem Brian is concerned about) is if the old hardware is given a fresh install of new OS and then placed back on the network. the new OS will by default use the old hardware's original MAC. – quack quixote – 2009-10-05T23:27:38.930