MS has handed you a nightly update change to your security settings.
Likely MS is now looking for "MS registrations and approvals" that Jon doesn't use as this site runs on YaBB freeware. Therefore this site's forum is now "forbidden" to you as a loyal MS user, for your own good of course.
It may be that a virus or other illness was attached to a picture or in something else referred to by another forum member that has caused the forum portion to be "forbidden" to you by MS as the bad thing was found there.
You need to write down the information that flashes up on the forbidden screen and then do some web searches on it -- nothing happens just to you, it happens to LOTS of people and the "how to fix it" becomes quickly known.
Step 1 is to turn your machine completely off, count to 10 and turn it back on again. Forcing a cold boot reload of Windows fixes many small problems and is always worth doing as a first step.
========================================
I downloaded and ran ClamAV on my Linux box, not because I need it but because Windows people need for us Linux people to not be transmitting viruses to you (we can't get them, but you can).
Scan came up clean, I am not a "virus pusher".