If the computer is being offered for retail sale at your local computer store or being advertised in a magazine for the general public, you can be almost certain that the computer is a Class B device. There are several things in a computer that can generate noise. These signals can be generated by the several oscillators found in most computer systems. Signals from the oscillators can interfere with the signals we want to receive. Additional digital noise can be generated by the video monitor circuits.
Computers also use switching power supplies.
The monitor has a separate power supply, plus sweep and high-voltage circuits that can also generate noise. Interference that is intolerable with an indoor antenna might go away completely if you erect a good, outdoor antenna. In addition to reducing the amount of signal picked up from the computer, this has the benefit of increasing the amount of desired signal. Most computer systems have shielding to minimize the radiated signals. The shielding can be external, provided by a good metal case and shielded cables, or internal, provided by ground planes or metal partitions on critical areas on the printed circuit boards.