start -> type "cmd" (for vista) and press enter -> type: net stop "automatic updates"
automatic updates is off completely and wont shut you down. Once you reboot manually it will turn itself back on.
i do this often for work. we have to keep automatic updates settings in a certain way, but on days it happens it will be like 3pm and it will try to shut down my computer. the work i do i often have a server running locally that takes ~15-20 minutes to boot up, so a shutdown is no easy task, especially near the end of a day. i just delay it and then later do it myself. its a pain because the xp machine i use for work, it gives you the option "reboot now" and "ask again" .. thats it... and ask again means "ok, we'll bug the shit outa you every 10 minutes - and oops? did you leave for a few minutes to talk to a coworker? we'll just shut down for you"