How to authenticate user when he enters password that should be the his windows password?

Hi,
I just begin creating a website for an organization. First page to be displayed in the login page. I dont have any knowledge in ASP.NET Security. On Login page, i want to display UserName field as disabled with the useralias of the user who currently logged into Windows. Password user has to provide and it should be that user's windows password.
How do i validate that password that user has entered is his windows password?
Thanks,
Pratap
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