In my opinion the 1% comes from problems caused by tweaking or hardening the system. I can do a clean install of any version of Windows and then run Windows Update repeatedly, until every available update is installed, except for some optional updates that I may choose not to install and nothing will get broken. Generally the worst that will happen is that I may occasionally get an error when installing an update and this is almost always fixed by rebooting. On Vista systems, you need to manually install a few updates, or Windows Update will get stuck at "Checking for Updates..." But once the updates are installed, Windows Update works just fine. Not that I use Vista too much these days.