It’s not acceptable to restart the user’s computer, losing their data, without their permission
Windows XP downloads updates for you automatically, then installs them, then asserts your computer has to be restarted. You can click "restart later" but the assertion is simply repeated later. That's quite annoying, but I suppose one can get used to it.
But the most outrageous thing is that if you don't click "restart later" in time, it restarts your computer for you. If you take a short break for the computer – go for a coffee – and come back, you find your computer has no open windows. What about all those websites and documents and other things one was working on? Gone.
Windows really is so bad, one shouldn't use it.