why is this happening?  
when i plug in my mp3, it shuts off.
not good psu or what? i have neo 550
help!!!

Need a few more details than that...
 
Is it only the MP3 Player?
 
Are you pluggin this into a front case USB port?

i tried the 2 on the front of the case, and all of the usbs on the back of the case, it is not only mp3, it does that with my external hdd, flash drive, everything! it didnt do that until last week, i never opened the case after i built it since it is only 3 week old

Before you plug anthing in touch it to the metal on your case to ground it and remove any static. This time of year is dry and there is alot of static. Just because you dont se it or dont hear it dont mean it aint there. Mine does the same thing but I found grounding before plugging in works every time.

I believe the problem isn't necessarily static, as much as the USB headers have something crossed.  Are they the manual kind where you have to put each pin in its place?  It would only cause a short when you plug something in.  I'd unplug all headers, then plug in your mp3 player or flash drive to the motherboard ports and see if all is well.

check your usb device current rating and compare to the motherboard usb head current rating, if it over the motherboard spec, the pc shut off to prove future damage. You can also checck your psu rating too.