The problem I have is as follows: I connected the graphics card to one monitor first with DVI and a second one through a DisplayPort to DVI adapter. The computer has a Standard VGA adapter and an Intel Graphics HD card embedded, and when I connected the second monitor it worked, but it was duplicated. Going to the monitor settings there was not possibility to have a second monitor detected, albeit the output was there on both the monitors, in the sense that it was exactly duplicated. My OS is Windows 7 Professional, if it may help.
Hello to everybody and thanks for your interest in my question.
Actually the computer was a friends' one and the problem was so simple: I don't get why, in fact the drivers of the Graphics card, ATI FirePro v4800 HD or so, was not installed and it seems that the default one provided by Windows doesn't let you extend the display.
Thanks again to everybody.