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.