Windows Update destroys my internet connection

The first step is find out which update is causing the problem.

Go to Control Panel > Automatic Updates and select the option "Download updates for me, but let me choose when to install them". Then when the update is initiated you'll get a dialog asking if you want "Express" or "Custom".

Choose "Custom" and then uncheck all but one of the updates (make a note of which one you leave) and then allow the update to proceed. If your internet connection remains working then proceed to the next one in the list. If your internet connection fails you've found the culprit.

Apply all the updates apart from this one. Then try and find out why this update is causing the problem.