Hi. First of all, you MUST have the official Synaptics Touchpad drivers from the Dell website installed, else the 'Dell Touchpad' tab will not show in the mouse configuration window!
Once the drivers are installed, open Control Panel --> Mouse --> Dell Touchpad.
Click on the large screen with an arrow, which will open the touchpad configuration window.
Click on 'Device Select', and right at the bottom is a box which you can tick to 'Disable Touchpad/Pointing Stick when a USB mouse is connected.
As soon as you tick that option, a message will pop up in the taskbar, confirming your selection.
You can make doubly sure however, by also choosing 'Disable' under the Touchpad' heading in the same window.
Be sure to click 'Apply' once you've made your selections.