The reply from David was correct, my video card isn't supported anymore by the latest xorg server (1.6). So I return to Mandriva 2009.0 and the problem was solved. It's a shame but I understand. I will buy a new video card and try with 2009.1 later. But, just to mention, in the couple of days that I use 2009.1 I could watch videos without problems. The real problem was with the graphical interface responding slowly. Like waiting a couple of seconds for the kicker menu to appear, etc.