Vista has a habit of installing its own generic drivers for some hardware even though you install ones for it yourself. (Especially if they are not WHQL'd)
Go to Device Manager and look at the drivers tab of the mouse and make sure it is using the Logitech ones you installed. It might just be a case of clicking Update Driver and point Windows to where the driver is installed.
Also make sure that you uninstalled your old mouse and any software for it.