you could offcourse rma the card if its wining. ive got a Enermax psu my self a NaXN and it does not wine at all.
but that still does not solve your main problem.. i would say first try windows 7 just to determine if its a windows issue it self, or if it is realy hardware related, if the cards works perfect on your brothers system, then you know for sure its not the gpu on its own.
could be a bad psu, or even a bad pci-e slot.