well most of the time if the power supply is bad the computer (fans, harddrives, cd-roms etc.) wont make any movement / noise. if you can hear the computer start up, but no display, try removing the modem card if he has one. if that doesnt work try removing any other pci cards that might be installed (except if he runs a video card). if there is still no luck after that, i would guess ram. but as far as a computer running with a bad power supply.... only a power supply tester can find out. you can normally pick one up at a local computer shop (not a large retail such as best buy or circuit city from what i know) for about 10 bucks. hope that helps