screen goes black when i uncheck dx10

Are you running Vista or XP and how much memory do you have.
Your card is using 512mb of the memory in your machine.

Try lowering all the slides, especially autogen, shadows and AI

Thanks for the reply.I have 4gb memory and dual core 2gh processor.
Problem is when I uncheck the dx10 preview I get black screen for all aircraft including default ones.I can hear the sounds when the black screen appears.Funny thing is I can use the game fine with dx10 selected just that all add on aircraft come up white.This is a brand new machine bought especially for playing fsx so this is a real downer.The graphics card is a nvidia 8600m GT and Ive tried using n hancer with various AA settings but nothing seems to work.

APOLOGIES BUT TO ANSWER YOUR QUESTION I'M RUNNING ON VISTA AND THE CARD ITSELF HAS 512MB MEMORY.

Try getting new drivers for your video card, usually the ones that come with the machine are old.
Try without enhancer first (I'm an ATi user, not too familiar with that).
Don't forget to clean out the old drivers before you install new ones.

Ok ,updated all the drivers ,cleaned out hdd and defragged,even wiped down the screen with polish.No change cant get any addon aircraft to work and these are all new fsx compatible files.Cant figure out is it the software or hardware.Open to any suggestions from anyone who had similar problems .This is a brand new xps machine with fs9 the only other software i run on it.Thanks to radarman for help todate