Power resistance on motherboards

I was at this client’s location looking at a computer that had been loaded with additional devices.(CDROM READER & DVD and other stuff) When I first booted the computer all the icons changed color or there was no icon just an horizotal line for an icon and it was not responding to the mouse. I could not make up the problem because I was told that it had enough RAM, Windows has been reloaded and no sign hardware conflicts. The bottom line was that another tech had tested the motherboard and the problem was high voltage. I could not find the tech to ask how do I go about testing the hardware since I really did not know how to diagnose the problem. can somebody tell me?