The switching mode power supplies used in personal computers are major sources of harmonic currents. Measured and calculated waveforms and harmonic levels of input currents of various types of personal computers are presented. Harmonic currents of a group of personal computers reinforce each other. Neutral currents resulting from the addition of triplen harmonics are analyzed and recommendations are made to safeguard against potential problems. The impact of personal computer harmonic currents on distribution transformer losses are explored. Harmonic currents result in the overheating of transformer due to excessive eddy current losses. In addition, the influence of three-phase transformer connection on the distribution system neutral current is examined. In the past ten years several methods of power factor improvement through harmonic elimination have been developed that use high frequency switches to actively shape the input current. A new single- phase harmonic reduction circuit based on the voltage- doubler circuit is presented that uses a switch operated on line-frequency. Significant reduction in the THD of the supply current is achieved with the proposed method. Theoretical, simulation and laboratory waveforms of the input current and its harmonics are presented. The advantages and disadvantages of the proposed circuit are outlined.