Software to test a PSU

Some (read: medium/higher end) Mother Boards come with hardware/software that allows you to log your voltages in windows. But you have to have the right MB first.

The voltage sensing hardware built into the motherboard is not very accurate and can't be trusted without first caibrating it and the software that reads it against an accurate meter, over the full range of temperatures the computer operates at. There are meters that will allow their measurements to be logged to the computer or that can remember the highest and lowest measurements taken over a time period. The official tolerance for the positive voltages is +- 5%, but supplies should usually stay within +- 3%.