Computer turns on but won't boot up...?

[Josh C]

I've starting using my old emachines T2825 PC (hasn't been used in a year or two) since my laptop got stolen. It's been over a month now, and besides running a little slow, I haven't had any problems. But then I came home the other night, I found my computer was off and wouldn't turn on at all. I opened the case up and there was a potent metallic smell coming from the PSU, so I decided to replace it. Now my computer will turn on (fans are running, etc.), but nothing shows on the monitor, and there are no beeps or sounds. I've made sure everything is plugged in nice and tight, and everything seems to be in its place. There was some dust buildup, but I've cleaned most of that up. So what might be wrong? What can I do to troubleshoot?