Go into Control Panel > Printers and Hardware > Mouse
Under the first tab (Device Select), select "Disable Device" under Touch Pad.
That's how it is for me in XP. I'm not sure if Vista and 7 look different, but it should be somewhat similar paths to the Mouse options.
*EDIT*: Actually, are you using an external mouse? If so, there's a separate option to disable the touchpad when you have an external mouse plugged in.