In IIS Manager click on your site. You need to be "in feature view" (rather than "content view")
In the IIS section of "feature view" choose the so-called feature "authentication" and doulbe click it. Here you can enable Windows Authentication. This is also possible (by i think in one of the suggestions in the thread) by a setting in the web.config ( ...)
But maybe you have a web.config you do not want to scrue too much around with. Then this thread wouldnt be too much help, which is why i added this answer.