On Windows 10, go to All settings>Devices>Touchpad and the switch the Touchpad from On to Off.
If you have an external mouse then uncheck Leave touchpad on when a mouse is connected..
On Windows 7, Go to Control panel>Mouse>Dell touchpad image and the Pointing devices open
Click the box Disable Touchpad & Pointstick when USB mouse present.
I you don't have that option the install the Dell touchpad drivers.
You could also disable the touchpad in the Device manager on both Windows 7 and 10.