Microsoft's zeal to ensure that Windows 10 is the most secure operating system they have ever made has now turned into a major problem - IT AUTOMATICALLY REBOOTS ITSELF!
Unlike Apple's OS X operating system, Microsoft Windows 10 does NOT save the state of open applications or documents, so they will automatically restore themselves after the computer reboots. So if you did not save your work before it rebooted itself, the work is lost.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips