Bob can project on the walls, floors, ceilings of the Castle and can pick up electro magnetic frequencies. You just need a wireless router in one of the neighbouring houses too far away for Harry to disrupt to him to patch into the internet, and a nearby cell tower to patch in for cell phones.
Harry doesn’t need a computer, the Castle is now a computer. Harry has entered the silicon age. Literally.
Bob is capable of multi-tasking (at least at watching porn) so he should be interact with multiple users as his own local network for the castle users.
Initially his projections will be blue BUT we have seen him use other colours and Blue, green and red should be possible so he can go full colour. Harry will be so excited when the Mandalorian streams in the Dresdenverse (it’s currently late 2014) so several years to go.
Sound reproduction may be a problem, Bob isn’t a mimic and has only ever reproduced his own voice. In doing so it is either telepathic, of he is manipulating the air molecules to create sound. However as he can interface with a loudspeaker that ceases to be an issue.
Loudspeaker technology is old, Harry uses an old style radio but decent valve driven speakers date from 1912, over a hundred years old, so it would be relatively easy to custom build speakers using that tech (older by thirty years than the Munster mobile and amplifiers use valves today) with Bob acting as a carrier wave for them (he communicated via Harry’s office radio early in the series), Harry should be able to get full sound reproduction, even if Bob cannot provide it directly.Using a magic circle would cut off Bob’s carrier wave, but the tech would be old enough to likely hardly ever burn out because of Harry.
Whilst Harry can speak to Bob to dictate things it might be possible to rig up a keyboard. Get a slab of Castle Stone and have Bob project a keyboard touchpad on it. Bob should be able to discern the keys being touched and make the corresponding alpha numeric notation.