You can change your MAC in Windows but it is a hassle and buggy, just use any linux distribution and download MacChanger. Very easy, but not needed for what you want, also you would need to change that MAC at every bootup.
When it comes to identifying you that MAC is the least of your problems though.
You want to achieve stealth.
For that you change your OS as it talks way too much.
Think about it, when you visit a page, your OS, browser, screen resolution, browser history, computer name, installed plugins are all visible, without even allowing JavaScript. All that and more, for anybody you connect to and no way to stop it.
If you include JavaScript, I can follow your mouse across the screen, see where you click and what you type amongst other things...
So, don't try and alter your computer, change it completely!
The easiest way is installing another OS to boot from, next to your Windows installation of course. This would of course only cover 1 stealth account.
Also, use proxies and
check them at whatismyip or something like that.
A proxy does not by definition hide you IP address. Not all anonymous proxies hide the fact they are proxies but a high anonymous does. Steer away from free high anonymous proxies with keep-alive feature though. If it is too good to be true...
I hope this helps.
