Devices that emit strong electronic fields can also interfere with speakers, subwoofers, wireless mice / keyboards / whatever. As far as a microwave in the same room as a computer, I doubt that'd harm anything. I had a microwave in a very tiny room with two computers for a while year and I never noticed anything bad happening. On the other hand if all of your computer components are sitting on top of your microwave while you run it, that probably isn't a good thing.