You got it all wrong.
I mean that when the user has entered the captcha and hit the "I accept" button, how will you check that he entered the correct captcha ?
"I accept" button calls this function on click:
SignUp.DoPost()
Now how would you know what to do on "DoPost" method ?
Unless he fills the real form on the Windows live's website, he can't submit the form and create a new account.
That's what I meant.