In Windows 8
Right click in the Task Bar or Ctrl + Alt + Delete to open taskbar menu option. Then click in the performance tab and watch in footer of Task Manager.
Method 2: in Command Prompt (CMD)
To Open Command Prompt Go to run / Press Windows Key + R, then write cmd and Press Enter
Another Method: Using Third Party Tools
In Windows 7 and Vista you are able to use Gadget to see the System Uptime or Boot Time
