We already have minor, controllable variations of this.

When I was younger back in the days of Windows 95 / 98 I had this program called Catz 3 and Dogz 3. You had little cats and dogs which you adopted as pets and they played together. You had to feed them etc..

Noadays I have the sims - If you give your sims free will, and don't give them instructions, they take over their own lifes. We are getting closer to this happening. All it takes is for a few of these "Sims" to decide they are taking no more shit.

However, I think its more likely someone will write a virus, and the virus itself takes over the PC, and can't be controlled. The virus could then spread to other equipment.