here is the main idea... A MAC address is supposed to be Hard Coded into the ROM on the device itself...
That is why MAC address were once good for security purposes...
Because they were intended to Not be able to be set from software.
At least that was the original intent, The Theory was no two devices would ever have the same MAC address.
and routers could use this ID for preventing access to networks...
If the MAC address was Hard Coded into the NICs ROM (Unlike an IP address that is alterable from software)
as proposed in the original THEORY the only way to change a MAC address was to get a new NIC... 
In the old days Flashing the ROM on a NIC might set a new MAC address...
But in THEORY it still should not have changed
It was intended to be BURNED INTO the HARDWARE Permanently 
...
What a great security measure! You could as a network admin POSITIVELY ID a specific PC / NIC (like a drivers license)
http://en.wikipedia.org/wiki/MAC_address
"Although intended to be a permanent and globally unique identification, it is possible to change the MAC address on most of today's hardware, an action often referred to as MAC spoofing"
Over the years this really excellent plan was shot full of holes...
First they allowed changing it during a BIOS / ROM flash. Later by Driver and eventually by Registry.
Now making it totaly useless to anyone at all...
So why do you wish to change this totaly useless number?
