You can find out by right clicking on my computer > properties > system information > more info. This will give you information about how many times windows was rebooted since installation (in days), what time each reboot occurred (in hours), how long each session lasted (in minutes) etc. . you can also view these details from windows task manager as well if you click start button > run. . . type taskmgr into search box then select task manager from results list.