I do not remember if I got this info here or not so I thought I would post to help others that may have this problem. For those running enb bloom and getting ctd's with alt-tabbing what fixed it for me was this: http://www.dll-files.com/dllindex/dll-files.shtml?d3dx9_26 I have so far not got any ctd's as I alt tab or go into menus to make adjustments. This is to replace the d3dx9 from enb in your FSX directory. For me it was getting so bad I could not even go into a menu once without getting fatal error ctd. I cannot live w/o enb. Once you try it and tweak it for yourself nothing will ever be the same again.

This is old news, I tried this a long time ago and it fixed the Alt Tab issue, but If you fly for say and hour or more and try to do the Alt Tab you''l get a crash...or at least I do. None the less thanks for posting this and I hope you have a better experience then I did!

Ok I take that back, it worked and now it does not work. Go figure. The CTD's only happen with the NGX. For anyone that has it working what Nvidia drivers are you running? I am on the 280.26 driver, should I try a different one? ENB aside, perhaps there is an all around better driver people are using anyway.

I get CTD with Alt-Tab as well.I now only use Alt-Enter to get FSX into windowed mode.Then hit Alt-Enter again to get back to fullscreen mode.No CTD with this method.

From what I have read, alt tab is to be avoided at all cost in FSX. The best way to handle task switching in windows with FSX is with the alt enter to go from full screen to windowed mode, and supposedly, Nvidia claims that alt enter should not be used and that they prefer using the FSX menu to switch between screen modes. That being said, I have mostly used alt enter without any ill effect. The above advice applies to FSX with or without ENB mod, but speaking to the OP's topic, I have tried numerous times to fix the full-windowed mode CTD with ENB mod without success, and have no resigned myself to accept that there is nothing I can do to fix this issue. I believe I tried this method as well, and I still had a crash. Like the OP says, it's hard to guess if there will be crash, it's almost like russian roulette, and i'd rather not have that uncertainity playing in the back of mind when I'm trying to enjoy FSX, so I have just given up on ENB. I did find a somewhat nice replacement effect with "Shade" but unfortunately that effect isn't quite as noticeable and only applies during the dusk or dawn early/late hour time transitions.