Hi there,
This indeed is a very interesting question.
After dwelling on this for well over 10 minutes, here's my take on it.
To me, the computer is an artificial brain and so we could also ask, " Are our brains musical instruments" ? The answer would be no, but without our brains there would be no music. I see the computer more as a storage device,(like our brains), and also a controller. It allows us to control the information stored in it's brain. Sometimes making music,other times might be drawing maps, whatever. It becomes a musical tool once a musician gets involved.
The main difference is this I think:
A computer has the potential to create music but without a musician, it is just a computer.
A guitar or saxophone is still a guitar or saxophone even without a musician present,... or is it ? ?
Very thought provoking, Gordon.