Initially it seemed to be a grapcis card failure with the original AMD V7900 card (6 blinks on the power-button).
CMOS was reset (unplugged and drained power, pushed yellow button for 30 seconds)
Exchanged the AMD V7900 card with AMD W5100.
At start, the HP logo appeared on screen, but then the booting stopped with a blinking cursor at the upper left corner.
I'm able to get into the Start up menu/BIOS by pushing F10, and resore settings, but this does not change the situation.
The new graphics card seem to be working, since information appear on teh screen
Any ideas how to proceed?