Since they are different devices, I would not expect a different driver to work on another device. Did you have some indication the Synaptics driver would be compatible with the Alps touchpad?
To remove the device and its drivers, you probably need to go into the Device Manager (Devmgmt.msc) and uninstall the device. Make sure and check to remove all drivers. Then reinstall the Dell drivers.
You may also have an option to rollback the drivers, which you could try first.