So, FSX crashes when minimised and I know this is usually an issue with it being in fullscreen, but this time it was windowed. It keeps doing this and, since, for reasons I'm really not interested in explaining, I can only have one monitor. This is an issue because of littlenavmap. I need to see it. So, I'm going to try verifying the game files and seeing if that works. if it doesn't, I'll reinstall. If that doesn't work then I'm screwed. I did see one post with someone who had the same problem and they deleted an fsx.cfg file but I can't find this anywhere.

The AVSIM CTD Guide will give you a lot of information about where files are located and what to do in the event you have a crash.  See link in my signature.

You should have FSX with SP1 and SP2 installed or FSX with Acceleration (which includes SP1/SP2).  On page 10, AVSIM CTD Guide, you will find instructions on how to fix almost every crash/freeze with FSX.  The first recommendation is renaming the fsx.cfg to fsx.off as this usually fixes every crash. 

I tried to replicate your crash in windowed mode and could not.  I normally fly in windowed mode as I get better performance. 

12 hours ago, Jim Young said:

The first recommendation is renaming the fsx.cfg to fsx.off as this usually fixes every crash.

Where is fsx.cfg located?

It is hidden and why I recommended looking at the AVSIM CTD Guide.  It takes some time to explain how to unhide files and folders in Windows and, then, once completed, explain to you exactly where it is located.  The guide provides pictures.