I have updated to W10 a couple of months ago and I have just installed FO:NV via Steam and it doesn't start. It loads the launcher, buy when I clikc "Play", launcher closes and opens again, without opening the game. I tried running FalloutNV.exe directly, but it opens the launcher again, I click "Play" and launcher closes without opening the game (but now it doesn't restart). I tried with admin permissions and compatibility with Vista/7. Tried unistalling/installing again, verifying cache... I played without problems with W8.1, but now I can't go back... I also tried NSVE and 4GB, it still pop up again launcher. Also installed in default Steam directory, since installing in D: caused application error 5:000... Which where solved by installing in default directory, but it doesn't run. Updated BIOS, Nvidia drivers, renamed FalloutNVLauncher to something else to skip launcher, checked NTFS permissions, I don't have mods and don't use games boosters and related. IDK what more to do... PLEASE HELP

Check this directory: C:\......\Documents\My Games\FalloutNV

Do you see Fallout.ini and Falloutprefs.ini? Try deleting them if they are there and try to start the game.

Is FalloutNV.exe and FalloutNVLauncher.exe in your anti-malware exceptions list?