Dual mode allows you to tap in the top right hand corner to activate a 
set of function buttons on your mouse pad.  When this happens other 
movement is disabled.  Quite often I enable these buttons by accident 
and then I have to turn it off before I can continue.

Dual mode can be disabled as follows:
1) Select Mouse from Control Panel.
2) Select the Device Settings tab.
3) Select the Settings... button.
4) Select Tapping and then Dual Mode from the option tree on the left.
5) Deselect "Switch to Button Mode by tapping in the upper-right corner."
6) Click Apply and enjoy your new found freedom from dual mode.