Symptoms

Shortly after starting Windows the Synaptics touchpad on my laptop stops working.

Cause #1

You accidentally pressed a key to lock the touchpad.  When Windows starts it loads a Hotkeys driver (= special function keys typically found on laptops) that allow you to change the brightness of the laptop screen, or the audio volume of the speakers.  There is also a hotkey to lock the touchpad.  When Windows starts the touchpad will initially work for a few seconds or so, until the Hotkey driver software is loaded into memory and locks the touchpad.

Solution #1

Press F5 to lock/unlock the touchpad.

Alternatively press simultaneously Fn+F5 or Ctrl+F5 (or Fn + F9 or Ctrl+F9 on some models).

Cause #2

The touchpad is stuck in scroll mode.

Solution #2

Swipe on the touchpad from right to left. This should bring up a sidebar on the right hand side of the screen and show the time and date in Windows 8.  The touchpad should now work properly again.