Plugged in computer it made a popping sound, smelled like smoke, then would not turn on again, what happened?

Okay, so I will try to keep the back story on this short, although it's been long and painstaking. I had an older desktop that was built for me by this computer company in 2003. Although it was old it ran well. About a year ago it caught a nasty virus. I shipped it to the company (lifetime warranty) to be fixed. They fixed it however they packaged it improperly and the machine was mangled in shipping. Yes it was insured, UPS will not pay because of improper packing. So after many battles they agree to send us a comparable machine. It was not. It was slow and it crashed all the time. It sucked for lack of more eloquent terms. So after threatening the company with small claims court they sent me yet another "comparable' computer. So I unplugged the old computer which was powering up just fine connected to a power strip on an electrical outlet that is perfectly functioning and updated blah blah you get the point. I plug in the computer i.e. the most recent piece of junk they sent me (sorry I am so bitter) to the power source only. Not other hardware like monitors or keyboards or anything. I turned on the power switch and pop, pop, burning smell and then dead. Whoa! What happened? I plugged the old computer back in it turned on and is working fine, well as find as it was anyway. Someone please explain to me what went wrong.