Windows Reliability Monitor
Your PC may be behaving badly, even if it doesn't appear so outwardly. But fear not: Windows Reliability Monitor is a tattle-tale who isn't afraid to spill the beans. It shows all problems that Windows has encountered in a chronological chart, which you can sort and click through for more information on a day-by-day and case-by-case basis. The tool's especially handy while you're tracking down trouble programs that could be the cause of weird crashes.
To open the Reliability Monitor, open the Control Panel and head to System and Security > Review your computer's status and resolve issues (under Action Center) > Maintenance > View reliability history (under "Check for solutions to problem reports").