Computers emit radio waves, by varying the processing upon the computer you can influence those emissions. Thus you are able to control those emissions and produce a radio signal you can pick up upon a standard radio.
These EM fluctuations are the emmissions we are talking about here.
Another likely source is the power supply, which has coils by the dozens and will react to varying loads of computational work. The cheap ones are already all over the radio spectrum even when you don’t want them to be.
Choosing which part of the computer is a matter of choosing the frequency which dictates the length of wire necessary and which parts can oscillate at that frequency.