Shaun o said:
It maybe the old video drivers have not been removed.
I would check that the Nvidia driver is still not there on the system.
And also the Ati driver from your card you gave your friend to use.
Just a thought, if you find his card works in your system.
Also double check the specs on the new Psu he bought make sure it can provide enough amps on the twelve volt power rails to drive the new card. It could be the cause, but I doubt it but check anyway.
Should work with both ATI cards, will try to put the GTX 550 Ti back in later aswell. Reinstalled windows after implementing the new card aswell as installing the latest drivers. But with no luck.