I downloaded the HP Alps touchpad driver sp40715.exe encoding
product version 7.2.1721.112/7.102.172 Rev.A (ALPS Touchpad Driver)
and this driver installed fine on my G550 Lenovo. It runs in the system
tray and can disable the touchpad under the 'Other' tab.

This driver does not seamlessly disable the touchpad when a USB mouse
is plugged in, but does the trick in a few simple clicks.