Windows Vista decides it's time to configure user's computer, and a reboot was necessary. All by its own. And you don't even have an option to delay the reboot at all.
Windows Vista's auto update setting is believed to be enabled by default, and if users have Vista's auto update on, their computers are subject get configured and rebooted at time Microsoft wishes, users have no option to delay such system reboot. In contrary, Firefox's default setting is to download the update, and the update will be applied at next relaunch of Firefox application.
Such method of rebooting users computer without giving users a chance to manage a convenient time is extremely unethical.
Just for the comparison, Mac OS X's System Update does not reboot (by itself) the machine you. It will give users chance to close and save documents first, and let users to reboot the machine. The default setting (preference) does not even download updates in the background.
Note: I don't have Windows Vista myself.
Watch the video report: Tech Shakedown: Vista forces reboots (Cnet News.com, 3 August 2007)