It happened to me more than thrice that suddenly, usually while browsing the Internet (with Firefox), a page freezes in the loading state, without ever finishing loading. No network issue is reported.
Then, my Skype completely freezes (edit: but yesterday it didn't do so), although Steam works still fine, and I can send and receive messages there. Although I can still launch processes, after a while, no additional processes can be run at all, and Process Explorer stops updating the process list. However, I am still able to close frozen applications (including Explorer), open windows of (some) previously started processes, and kill processes via Process Explorer. Programs which can still run are also able to open files. Even when I close all possible applications, the problem still persist, and even attempting to shut down the system or logging off does absolutely nothing. Only switching users does something - turn my screen black forever.
Eventually, I am forced to hard restart the computer, as even the power button on the computer doesn't send any signal to start shutting down the system.
I haven't noticed any process taking too much CPU in Process Explorer during the issue.
There were no updates to the system recently, aside from regular Flash, Java, and Avast updates (Windows Update is turned off), and also one NVIDIA update, although I think the problem may have started before it.
Edit: Just after downgrading NVIDIA to a version from February 2015, it happened again, though Skype didn't freeze this time.