Just incase your Trackpad is already acting strange and you would want to disable it when a mouse is connected, this guide is for you.
First, you must ensure that your Macbook is running Snow Leopard, I believe other versions of OSX have different ways of disabling the Trackpad.
First, go to Applications then System Preferences.
Select the Universal Access then go to the Mouse and Trackpad tab.
Tick the option Ignore Trackpad when Mouse is present.