How is windows authentication secure and trusted

In web applications, an hacker can brute force attack passowrds list and some how hack a less secure system.
 
If the same web applications use windows authetication, using domain name, user account, password, cant a hacker try that.
if windows authetincation is perfect, intranet applications which are specific to a company use windows authentication, someone outside the domain cannot login.
Cant a hacker spoof this. 
Which one is secure for web applications: forms authetications or windows authenctication.
( My web application needs windows authentication to  part of the application)

Or am i just getting the defintion wrong for windows authentication? if it is can some one differentiate which one to use where?
here is Mine
Forms authentication: i use it to check the user logged in , i create sesstion variables, and check for session abandon.
 
 
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