Deactivating the "DNS Client" under "services" using msconfig.exe seems to solve the problem. By definition this is deactivating DNS caching of windows, although I still have no idea why windows was accessing the internet every second once, ony to "cache dns" (as sysinternals process explorer showed the permanent traffic was caused by the DNS service, wrapped by shady svchost.exe). I rather thought it will cache dns everytime an URL is resolved anyway, but not when I don´t access the web at all. (Note: autoupdate is turned off)
So after deactivating this as described before, I got zero traffic as long as I don't access something explicitely. Phew!
So after deactivating this as described before, I got zero traffic as long as I don't access something explicitely. Phew!