Why won't the computer boot up?

My friend's mother is having trouble with a computer I let her have... there was nothing wrong with it before, but since she's gotten it, the computer sometimes won't boot, and sometimes it will, but never all the way... I've used all my resources trying to figure out what is wrong with it, but I'm at a loss, any ideas? (I'm not a computer fixer by trade, so I literally have run out of ideas... we've gone through a new fan, reinstalling windows, etc)