Missions Freeze

I am encountering game freezes with several missions (most I try). It looks my quad processor core 1 hits 100% and 'that all she wrote'. I have to ctrl-alt-del to get out. Memory seems locked at 2 GB. Any ideas?BTW: Phil's Blog refers to a Vista Upgrade but my it will not install: says not for my system. I still get the message that the color sheme had to be changed, etc. when starting FSX.Dell XPS 710QX6700 quad4 GB ramnVidia 8800 GTXVista 32 Home Premium