Shutdown or Bluescreen in FS X

I installed FSX successfully an could run it on my computer for about 5-10 minutes. Then there are three possibilities: i) windows xp freezes (I have to restart the computer the hard way); ii) the system just shuts down and restarts; iii) I get a bluescreen with the message "DRIVER_IRQL_NOT_LESS_OR_EQUAL" so I have to start the system by pushing the restart button of my computer.I am using FS 9 for several years without having any of these problems (I believe its my first shutdown or bluescreen with this computer system).I updated the NVIDIA driver, ran the Norton Utilities over my system and minimized the graphic options in FSX (everything to minimal") and tried different aircrafts - it doesn't matter -fewer or later the program crashed after at least 15 minutes.I am using a pentium 4 with 3,4 GHz and 1024 MB ram and a nvidia Geforce 6600 graphics card under Windows XP SP2 with all new Updates.Any ideas what could be wrong?