Eugene's observations

Pushing the ON button on a running computer has different results depending on the power settings.  It may cause the system to do nothing, enter sleep mode (the default), hibernate, or shutdown.  Computers setup by me are altered to shutdown when the button is pressed.  But what happens if you hold that button in for more than 4 seconds ?  It shuts down abruptly.  

This is sometimes the only way to shut it down if Windows is so messed up that nothing else will do, but it is exactly the same as pulling the power cord.  Hence, it is strongly discouraged as it is most likely to cause a corruption of some Windows files which will at best provide unexpected results next time you start (and forever after) and at worst prevent the system starting at all.

Why is that I hear you ask.  Well, these are a few of the things that have to happen in a clean shutdown:

• Checking to see if any user applications have not been closed yet (like an unsaved document) and prompt the user if necessary
• Stopping background services
• Waiting for the termination signal from services and applications that are open or running
• Flushing the cache to disk
• Writing log files
• All users are logged out
• Ending the shell
• Start installing Windows updates and tell the system to finish the update process during the next system start-up if necessary
• Send the ACPI shutdown signal (this is what turns the machine off)

If these things don't happen Windows is in an unknown state when it starts.  If you have to do this then at least try a repair immediately you start next time (SFC/SCANNOW) but try to avoid it at all costs.  The best advice if you have a non-responding or hung system is simply to do nothing as often times Windows will sort itself out eventually.  Failing that a Ctrl/C or Esc could help.  Try a Ctrl/Alt/Del and then logout as at least then your user profile record will be preserved.