Installation and uninstallation of the Synaptics Touchpad Driver is quite easy, call dpinst.exe /? and you'll get the supported switches.
But what to do if you do not have the setup-files but only a crippled Synaptics-Directory with the missing dpinst.exe - and you have to change the mouse driver ? In an unattended environment, 'rundll32.exe "C:\Program Files\Synaptics\SynTP\SynISDLL.dll",standAloneUninstall' won't help you because of its question about sure to uninstall.
I looked in SynISDLL.dll and searched for the string "standAloneUninstall". Some words around this one I found the word "SilentUninstall_CallerWillReboot" - this did it in my case. It is possible, that the people from Synaptics change the switch from version to version, but then it is also possible that you can find the right one right there.