Drivers are installed when iTunes is installed, but if for some reason there is a need to install the drivers manually they can be download below for most versions of Windows. These drivers switch between 2 USB modes and this will depend on how the phone or tablet was booted. The modes are:
iPhone:
Regular mode: Apple Mobile Device USB Driver
Recovery Mode: Apple Recovery (iBoot) USB Driver
iPad:
Regular mode: Apple iPod USB Driver
Recovery Mode: Apple Recovery (DFU) USB Driver
(Note: Apple devices give a different HWID for Recovery mode.)