You don't need to go tweak the machine's bios, if you only want the touchpad deactivated under X window. I've had the same problem, and solved it by using several X config files:
- one which activates only my usb mouse
- one with touchpad only
- one with both mice
Go in you X configuration directory (it should be /etc/X11), copy the main file (should be named "XF86Config") in, say, "XF86Config-nopad", and edit the copy. There should be a section named "Pointer" referring to the touchpad, and another to the external PS/2 mouse. Just delete the lines corresponding to the touchpad.
When this is done, test the new file. Close X (if not already done), then launch it via this command on the shell prompt:
# startx -- -xf86config XF86Config-nopad
If everything works well, the touchpad now does nothing. To activate it back, you have to close X, and launch it with the default config file, this way: