More on Windows Stealth Updates

I don't use Vista, and I am not currently using my XP box. So, does it mention anywhere in the Update controls, or in the EULAs, or the last minute text files, or the MS website, or anywhere, that MS can or will install code in stealth mode? Does it say anywhere that there may be required udpates that you cannot refuse? I do recall having updates for the update system before in 98 or XP, and it makes sense if they're fixing a vulnerability or improving something. But why on the sly?