windows vista- undo driver update?

hello. i have a new acer aspire 4730Z with an ALPS touchpad that was behaving erratically. It is running vista. i went into device manager and right clicked the entry for the touchpad and chose 'update driver' thinking it would find the newest driver and that might help.

It searched for a driver and installed it, i was given no choices along the way. when it was done, that entry in device manager ws no longer called alps touchpad it was called ps/2 compatible mouse.

I no longer have the alps icon in my system tray and no longer have anything but basic functionality of the touchpad, none of the extra features that were available.

how do i undo this update and get the alps driver back?

Thank you