hi just found the answer elsewhere thought id share it with you all,basically if you hold the button marked fn(usually different colour of txt)and press one of the function keys along the top i.e f1 f2 etc you will come to the one that enables and disables the touchpad,hope this helps someone.
beany
June 2009
I had lots of problems until I installed Alps Touchpad driver
Now when I plug my USB mouse in it automatically disables the touchpad