Constructing ClientCredentials from HttpContext.Current.User.Identity

In my web app I need to call a service using the credentials of the current logged in user (Windows Authentication).  The problem is that the service expects a ClientCredential object and my problem is how do you construct a ClientCredential object using HttpContext.Current.User.Identity?
Do I need to use impersonation, I dont want to use impersonation just for this purpose only.
...