Thanks for the rply. I upgraded to a 3R System iCEAGE 500 W Power Supply. It was tested on Hardwaresecrets.com and got a silver award.
Yesterday we tested another graphics card and the computer booted up. After that we pluged the 5770 Hawk into another computer and we got the same result. Computer does not boot. So I guess the card is DOA - at least I hope so :-)
Now I'll send it back to newegg and wait for a new one. I'll keep you posted.