Go to any Logitech icon on the Desktop. Or to Start, All Programs and see if there's an entry for Logitech. If there's an uninstall option use it.
Otherwise go Start, Control Panel, Add or Remove Programs and uninstall the Logitech software.
Obviously, unplug the webcam.
Restart the computer. Run the Logitech install sequence from the CD. Hope for the best.
If no joy you may have to edit the Registry -- not recommended.