As title. Spent hours and hours trying to figure out why Fallout NV wouldn't even launch. Uninstalled, Reinstalled, verified local files in steam, UAC is switched off, installed on my D: drive etc etc. Gave up. Reinstalled Fallout 3, checked it loaded which it did. Spent hours and hours installing a large mod list, only to find FOSE wouldn't work. More hours checking all the usual, no joy.
I finally figured out the issue just now, its Windows Exploit Protection. To resolve:
Right click start menu -> click settings
Update & Security
Windows Security
Virus & Threat detection
App & browser control
Exploit protection settings
Program settings
Add program to customise -> Choose exact file path
Browse to Fallout.exe
Scroll down, Tick 'Force randomisation for images (Mandatory ASLR) and make sure its set to Off.
Repeat for FalloutNV.exe, enjoy!