every time I try to open FSX it closes before I get to the main menu and says that fsx has "encountered a fatal error". Does anybody know how to fix this?

Have a look at a few other posts and find out what is causing the problem.

I've looked at other posts but they all seem to be about issues with windows 7 64bit and I have windows vista 64bit. Does that matter?

You need to check your event log and let us know what the error is.

re-installing FSX seemed to do the trick.