How can I deactivate my keyboard touchpad in Windows 8 on my Dell 5720 laptop?

If FN+F3 does not work, go to the Drivers and Downloads area of the Support section of Dell's web site, enter your machine's service tag number, and update the touchpad driver. Then if it still doesn't work, contact Dell Support.

Reference:
Dell: Disabling or Enabling the Touchpad Using the Hot Key