Unable to load missions

These are my specs. Pentium 4 2.8GHX,Memory 1.00 GB Ram, Radeon 9200 Video Card 256mb.Can anyone make any suggestions for me please? Since installation I have never been able to load the Missions completely. I get the loading complete and can hear the audio instructions in the backgound but it never proceeds further. I am able to load the Free Flights mostly but sometimes also get the same problems. I have also had crashing back to the desktop. Have tried uninstalling and reinstalling but to no effect. I have been using the default settings. I am not a computer buff so am somewhat at a loss. Could this be a problem with my copy of FSX or does the concensus of opinion suggest either my Video Card or insufficient memory is the cause. Thanking you in anticipation. Charlie