This morning i used normally the PC and after a while it remained with white screen.
after a reboot i note to see strange characters on the screen:
now i installed a fresh W7 x64 ultimate version and i note that i have the same problem. I noticed that everything is working just before i install VGA drivers (the same i installed the last time and were working, 8 month ago).
but i BIOS diagnostic i receive no error from VGA. The screen now is another time black