ok i got it fixed! it wasn't that hard. i had plans to undo all the new stuff i did by first taking off all the hardware off one by one (USB and PCI/e slots), then undo all the drivers just installed.
i started from the easiest, so the first thing i did was pull off everything connected to my comp by USB, which was an external HD, wireless mouse receiver, and a flash drive. that fixed the problem! then i needed to know which one it was so i put them back one by one and tested after each one. i found it was the receiver of my logitech VX wireless laser mouse. i really can't believe this one little thing prevents it from powering off...
sirma, i think you should try unplugging all you USB interface hardware first, then taking out the internal hardware one by one to see which one is it. if it's not any of those then it's probably not a hardware problem, it's a system conflict/command problem. then in that case, i wouldn't know what to do except dig around or read all the links PCeye posted