It's very easy to set the MAC Address on a linux box:
ifconfig eth0 hw ether 01:00:00:00:00:01

not so easy on windoze, you can use a ready made program:

either Macshift, by Nathan True, 
which is free, and comes with
source code released under the terms
of the GNU General Public License (GPL)), 
or SMAC (commercial, but easy to find)




If you are referring to SMAC, save your breath because it is
spotty at best, and in my testing with different corporate clients
the implementation of changing the MAC address made the DHCP fail.

This is from the SMAC site:

Quote:
SMAC took advantage of the NdisReadNetworkAddress function in the
Microsoft Device Driver Development Kit (DDK.)

NdisReadNetworkAddress(...) is called by the network adapter
driver to obtain a user specified MAC address in the registry.
After the driver confirms that there's a valid MAC address
specified in the registry key, the driver then programs the MAC
address to its hardware registers to override the burn-in MAC
address.


Overriding is not the issue, getting it to behave solidly and
stable is.

http://www.klcconsulting.net/smac/

These are the most popular methods of changing the MAC address,
however 99% of the time will fail in the DHCP resolution.

Quote:
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.

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

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

Under “General” tab, click on the “Configure” button

Click on “Advanced” tab

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

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.

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.

If successful, reboot your systems.


This method works only "in theory", as I haven't seen anyone reach
any degree of success with it.



Quote:
Method 2:

This method requires some knowledge on the Windows Registry. If
you are not familiar with Windows Registry, just use the SMAC tool
to change the MAC addresses, or consult with a technical person
before you attempt on the following steps. Also, make sure you
have a good backup of your registry.

a. Goto command prompt and type “ipconfig /all”, and

I. Record the Description for the NIC you want to change.

II. Record the Physical Address for the NIC you want to change.
Physical Address is the MAC Address

b. Goto command prompt and type “net config rdr”

c. Remember the number between the long number (GUID) inside the {
}. For example, in the above “net config rdr” output, for MAC
address “00C095ECB793,” you should remember
{1C9324AD-ADB7-4920-B02D-AB281838637A}. You can copy and paste it
to the Notepad, that’s probably the easiest way.

d. Go to Start -> Run, type “regedt32” to start registry editor.
Do not use “Regedit.”

e. Do a BACKUP of your registry in case you screw up the following
steps. To do this

I. Click on “HKEY_LOCAL_MACHINE on Local Machine” sub-window

II. Click on the root key “HKEY_LOCAL_MACHINE”.

III. Click on the drop-down menu “Registry -> Save Subtree As” and
save the backup registry in to a file. Keep this file in a safe
place.

f. Go to
“HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E9
72-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.

g. Go through each subkey that starts with 0000. Click on 0000,
check DriverDesc keyword on the right to see if that's the NIC you
want to change the MAC address. The DriveDesc should match the
Description you recorded from step (a.-I.). If you are not 100%
sure about the DriverDesc, then you can verify by checking if the
NetCfgInstanceID keyword value matches the GUID from step (c).
If there is no match, then move on to 0001, 0002, 0003, and so on,
until you find the one you want. Usually 0000 contains the first
NIC you installed on the computer.


h. Once you selected the subkey (i.e. 0000), check if there is a
keyword "NetworkAddress" exist in the right side of the window.

I. If "NetworkAddress" keyword does not exist, then create this
new keyword:

i. Click on the drop down menu “Edit -> Add Value”.

ii. In the Add Value window, enter the following value then click
OK. (See figure 4.)
Value Name: = NetworkAddress
Data Type: = REG_SZ


iii. String Editor window will pop up at this time.

iv. Enter the new MAC address you want to modify. Then click OK.
(There should not be any "-" in this address. Your entry should
only consist of 12 digits).

II. If "NetworkAddress" keyword exists, make sure it shows the
keyword type is REG_SZ, and it should show as
NetworkAddress:REG_SZ: . This keyword might not have a value at
this time.

i. Double click on the keyword NetworkAddress and the String
Editor window will pop up.

ii. Enter the new MAC address you want to modify. Then click OK.
(There should not be any "-" in this address. Your entry should
only consist of 12 digits.


j. There are 2 ways to make the new MAC address active. Method I
does not require a system reboot:

I. Goto Start->Setting->Control Panel, and double click on
"Network Neighborhood".
WARNING: Make sure you understand that you WILL lose the network
connection after completing step "ii." below, and
if you have a DHCP client, you will get a new IP address after
completing step "iii."

i. Select the Network Adaptor you just changed the MAC address.

ii. Right click on the selected Network Adaptor and click
"Disable."
Verify the status column for this adaptor changes to "Disabled"

iii. Right click on the selected Network Adaptor and click
"Enable."
Verify the status column for this adaptor changes to "Enabled"

iv. If for any reason it cannot be disabled or re-enabled, you
have to
reboot your system to make the changes effective.

II. Reboot your Windows system.

k. Once completing step j (if rebooting the system, wait until the
reboot is completed), go to command prompt, type “ipconfig /all”
to confirm the new MAC address.


Unfortunately in most cases the following procedure follows the
attempt to change the MAC address. Restoring the true HW MAC
address:

Quote:
Restore The TRUE Hardware burned-in MAC Address:

Remove the entry you added:

If you followed Method 1, then go back to the advanced properties
window and remove the entry you add.

If you followed Method 2, then remove the "NetworkAddress" keyword
you added in the registry.

Use step (j) above to activate the change you make.

Once rebooted, go to command prompt, type “ipconfig /all” to
confirm the original MAC address.



If MAC Address changes does not work:

If for whatever reason the MAC address cannot be changed using
method 2, make sure you restore the registry setting by following
the "Restore The TRUE Hardware burned-in MAC Address" instruction
above. If necessary, restore the registry you just backed-up to
get your system back to the original state. You can do this by
clicking on the drop-down menu “Registry->Restore,” and restore
your backup registry file.


It may work (as I noted there is a very small % that will succeed
in changing the MAC address), but the number of factors that must
coincide to make it work is just not worth even trying (unless you
are positive beforehand that those requirements/factors are
present).


In fact, under Windows XP, the MAC address can be changed in the Ethernet 
adapter's Properties menu, in the Advanced tab, as "MAC Address", 
"Locally Administered Address", "Ethernet Address" or "Network Address". 
The exact name depends on the Ethernet driver used; not all drivers 
support changing the MAC address in this way.

However, a better solution - requiring Administrative User Rights 
- is to pass over the System Registry Keys under 
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318}. 
Here settings for each network interface can be found. 
The contents of the string value called 'NetworkAddress' 
will be used to set the MAC address of the adapter when next it 
is enabled. Resetting the adapter can be accomplished in script with 
the freely available command line utility 'devcon' from Microsoft, 
or from the adapters context menu in the Network Connections 
control panel applet.