How do I disable the touch pad on a laptop when I plug in a usb mode

I am running Windows Vista on my Dell Inspiron 1501. I had some problem in the operating system and therefore I had to reinstall it and everything was working perfectly. But whenever I used to connect a USB mouse then the pc used to disable the toucpad automatically, but not it is not happening. I dont know what was the setting for this option before I reinstalled Vista. Can anyone inform me how do i turn on this feature again. Thanks for any information.

I think that you have to install the Synaptics Touchpad driver which you havent already in order to enable that feature. The drivers can be downloaded from the official Dell website or from this link, just search for it. After this driver is installed you will get the option to disable the touchpad. Just go to Control Panel > classic view > click Mouse > click the big pointer/arrowhead > click Device Select > look under Touch Pad, click the blank "dot" next to Disable and also checkmark the box next to disable the touchpad. Hope that helps.