I have Automatic Updates enabled and every now and then I'll get that little pop-up window that nags me to reboot to complete the update process. Usually, I ignore it and after 2 days my computer crashes. It crashes by freezing all the programs, including Windows Explorer, to the point that I can't even use CTRL-Alt-Delete (and my mouse lags).
I'm pretty sure it's Windows Updates that is causing this. So, for now, I've disabled Windows Updates.