"But in IIS is taking my Windows Authentication user.
I have enable windows authentication in IIS."
It sounds like you've answered your own question really, but for clarity: - if you're using Forms Authentication then you don't need Windows Authentication on top. If you enable this in IIS, the user identity of the Windows user gets passed on from IIS to your application.
I think if you turn off Windows Authentication in IIS and enable Anonymous Authentication instead, then your application will wait until a user logs in via your login form before it tries to identify them.