Missions not working in simulator X

Hi

I have loaded simulator X onto my new computer, after doing a couple of the
lesson flights I decided to have a go at a mission, this is when the problems
started.
Every time I press the go to briefing button the computer locks up, then
after about 30 seconds a fatal error message appears and the sim re-starts.

I have tried uninstalling then reinstalling the simulator with no success; I
have tried all variations of graphic setting again with no success. I have
just loaded the acceleration pack onto my system hoping that this would cure
but again the game still restarts as soon as you press the go to briefing
button.

Any help with this problem would be appreciated

My computer is as follows
Windows Vista Ultimate 64
Intel X9770 3.3GHz CPU
4GB RAM
Nvidia Geforce GTX280 Graphics card

Thanks in advance

Allan