Computer stopped powering up...

I have this old computer and a few weeks ago it stopped powering up, so after just waiting a few hours I powered it up and left it on since then. Fast forward to a few days ago it powers up but displays nothing on the screen. Replaced hard drive with a diff one and it still didnt work. Replaced vid card, still didnt work. Replaced motherboard, still didnt work. Replaced motherboard and vid card, still didnt work. Today I went to put it back to its default setup and it wouldnt even power up at all. Im guessing this is the power supply. So should I get a new power supply and will that fix the problem?

I think you've pretty much worked it down to the power supply. It sounds like you've done a fair amount of troubleshooting on it.