[willb]

Yes, so what Dave does is a lobotomy on Hal. With current technology you can 'hot swap' hard disks and other peripherals; while these aren't exactly essential to the running of the system, it shows that in principle parts of a computer can be added and removed with no effect on the core running parts.

But, there must have been a good reason for Hal to be included on the ship (it's been a while since I read it/saw the film, so I can't remember exactly what his function was) and if Dave killed Hal then surely he would have a great deal of difficulty maintaining all the functions Hal was there for.