why IE keep sending NTLMSSP_NEGOTIATE even though I switch to use login form ?

Question

I implement NTLM SSO, it works well, but when I want to switch to use login form, IE doesn't work:
1.SSO into my website
2.clear session and redirect to login page
3.key in username and password
4.submit and because the login i have bypass the SSO filter, so it works well in Firefox browser. But in IE, it still send a request with ntlmssp_negotiate message in it, but not a post request with username and password in a form. When this happens, user can not successfully login into the system.

So my question is : How can I stop IE from sending ntlmssp_negotiate request ???