I have finally cracked this issue - it suddenly occurred to me to try the oft-recommended (but never needed by me) art of deleting the existing FS9.cfg file (after taking a backup) and letting FS build a new one. And foof - the problem has gone away.
The problem must surely have caused been my weather programme, which offers the options of "realistic" icing conditions (which I didn't find at all realistic) which either failed to turn them off or left a corrupted entry in the cfg file.
We haven't heard from the OP for a while but if he hasn't solved it this is definitely worth a try.
HEALTH WARNING - this is not for the light-hearted. If your FS is carefully configured, you may need to spend time reconfiguring everything. I wasn't expecting to lose all my keyboard, yoke and throttle assignments and sensitivity settings and this took me quite a while to get it back to as I wanted. It might have been quicker to copy the section from the old FS9.cfg but I chose to go all out for a fresh file.
Best regards,
John