The other day I decided again to fly my JS41 out of EGPH, and it happened to be raining. All of a sudden mt cockpit windows all turned an opaque blue. After 2 days of internet searching, I found the solution. Some older aircraft, which the JS41 is one, don't process fsx rain correctly. The fix was found on flight1 in their library section called vcrainfix. If this happens to you, you can download it here: http://www.flight1.com/view.asp?page=library (http://www.flight1.com/view.asp?page=library) After I ran this installer, it fixed the problem, and also gives the option to restore the original rain effect files.
I have never had that issue when it rains.
What happens is the windscreen ices up completely turning opaque.
Switching on the windscreen heat solved that for me.