Hi there. This sounds like a problem i got with one of my systems.
Here it happens when some parts of the user filesystem in windows are manipulated. For example, connect a vpn network which then maps your C:\Users\<account> folder to a network drive.
What happens in the OS i can't tell, but the effect is that you can still move the mouse, see the desktop and already running applications will continue to work (depending on what the app does). But you can't start new applications, windows wont appear and processes won't start. Not even shutdown will work, the only solution is a hard reset.
If you experience this behavior shortly after logons (windows startup or screensaver login) this might be the same source.