Keep in mind your internet connection might not work if you change your MAC address.If you're behind a router that's not a problem but if you're not you might need to call your ISP and tell them that you need to reactivate their services on the given MAC.
First off right click on My Computer then go to Properties go to the Hardware tab and click Device Manager.

Now locate your Network Adapter right click and go to Properties find Advanced then Network Address change the value in there and click ok and close out of everything.
If you want to get your original MAC back just set it to Not Present.