Motherboard burning up power supply?

Ok, so about a month ago my power supply burnt up. I had had it for about two months before that in my new system (Athlon T-bird 800, Gigabyte GA-7ZX). I take it back to the place I got it and they said they would send it back to thier supplier and get me a new one. After a month or so of patiently waiting they finally get it in. So I go get the new unit, hook it up to computer and hit the power button, Nothing...Not a sound, the fan didn't start up or anything. I try unplugging and plugging everything back in a few times, then try putting it in my other computer, still nothing. So I smelled it and it was definetly burnt up. So today I took back to them, getting a bit ticked by now. And they say it's probably my motherboards fault, something wrong with the regulator? Wouldn't that burn up the motherboard though? Not the power supply? Is it really possible that a power supply could work two months, burn out, then a new one burns instantly when installed??? The supply that burnt up is a L&C Technology 250 Watt. Right now I have a 235 Watt that was in another computer, and it has been running fine for a month now (it's about 1 1/2 old). Are these guys just trying to screw me, or is it really my problem?
Thanks