Is there any way, when you’ve select Start/Shutdown (either shutdown fully, or restart), to stop the shutdown from taking place, by some kind of key entry or other action? I’m interested in Windows NT and later, up through XP.
none that i’m aware of…
but i did find this, relating to XP:
“When Windows shuts down, each running process is given 20 seconds to perform cleanup work by default. If a process does not respond within this time-out period, Windows displays the " Wait, End Task, or Cancel” dialog box for the process, which prompts you to wait for another 20 seconds, stop the process, or cancel the shutdown process."
you can increase this 20 second time-out period… but i don’t know of any process which will abort the shut down process once it commences…
sorry… i forgot to link:
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q305788
OK, was hoping for better news.
Well, you can always just turn the computer off, but then you have to deal with the annoying “Illegal shutdown–we will now force you to sit through ScanDisk as your punishment for not shutting the computer down properly” thing when you turn it back on again.
Which wasn’t what you had in mind, was it? 
But anyway, ShutDown and Restart don’t take that long–if you change your mind immediately after you click on it, why not just sit there and wait for it to finish cycling?
How about, when it says “Are you sure you want to shut down”, hitting “cancel”? Or am I missing something?
Obviously, there has to be some point of no return. You can’t expect to cancel it when it’s already off, certainly.
Unless you have a USB-enabled nuclear bomb connected & set to detonate on Windows shutdown never, EVER turn the power off in the middle of a Windows shutdown! Lots of really bad things can happen like corrupting the MBR or MFT. Things which can trash Windows completely.
Oh in XP you can shut down the computer–by turning it off (with the button, NOT the power switch!!). It then does the shut down procedure.