Use Windows username on Intranet forum posts

On our Intranet site we have several forums that anyone can post to and when a user creates a post we want to display their Windows username against it.  What currently happens is that if they're logged into Sitefinity then their Sitefinity username will be displayed.  If they're not logged in, which is most likely as there aren't many Sitefinity users, then the post says it's created by "Anonymous".

Can anyone point me in the direction of how we could do this?