Hello Anon, and welcome to Vista Forums.
The newsgroups here are mirrored from the Microsoft Public newsgroups. Since the newsgroups are public, other websites can mirror an access them to. When you post something in a newsgroup, it will show up at every website that decides to mirror them as well.
Only posts in the Vista Forums stay only here at Vista Forums, and can only be accessed by members of Vista Forums.
Hope this helps,
Shawn