My PC is faulty. What component is broken?

Hi guys,

I was wondering if you could help me. My computer suffered from a faulty PSU. Unfortunately, the thing i connected it to to get electricity was not an AVR like i previously thought. It would turn on before and would just completely die out in a few minutes. we all thought that the motherboard was broken so we replaced it. (But I did not reformat. I just installed the drivers of the new motherboard) After replacing, my PC still had the same problem so we replaced the faulty PSU. It turns on great now but the USB ports of the new MB are not running (They provide electricity but do not read the hardware that's in it) We then thought the new MB was faulty so we placed the old MB in the rig. The problem is the same when it came to the old motherboard;the USB ports provide electricity but don't read anything. What could be broken here? My brother has a theory that the processor is broken. Would it be possible that the processor is broken but i can still use the PC like i'm doing right now? and another problem is I can't uninstall/install anything now properly. Your help will be greatly appreciated.. :(