PC Lockup or fried motherboard

[Beaker]

Ive been trying to fix this friend's six year old HP Pavillion 500, a real old school machine with only a 60 GB harddrive and 512 MB. I tried a different AC cable to the back of the current power supply and the light blinks but none of the fans or anything turn on, and ive checked and reseated all the memory and cables and everything appears fine, I tried a new powersupply that came with a computer case i bought and still nothing. I noticed the motherboard didnt even have a video card socket so all the sound came from the motherboard, and she told me that her sound stopped working before her computer did, so another friend of mine told me he was certain it was the motherboard, but i also read on this forum about a lock-up feature which old computers have. Is it most likely the motherboard or something else?