Im having problems with fnv as in this run time error problem...I tried verrifying the game through steam, restarting my computer...
It will say

Microsoft Visual C++ Runtime LIbrary

Runtime Error
Program...\steam\steamapps\common\fallout new   vegas\falloutNV.exe

This Application has requested the runtime to terminate it in an unusual way.
Please contact the applications support team for more information.
And it says this when i reach the main menu...

I don't remember very well what this error was about. Needless to say, a runtime error is an error that occurs once you're running the program and it usually happens for one or two reasons, either the software is bugged, or two you're missing some components to make it run right, and usually this shows up when you have problems with the C++ redists.

You use TTW right? I would recommend going to their page, and checking which redists it requires, redownload them, and reinstall.

Usually the C++ error is because your d3dx9 file is broken. I've gotten it a few times....sometimes it would go away by quitting the game (which is odd but no complaints from me) but usually you need to replace the file.
I'd offer a link but there are tons,and you should pick whatever site you're comfortable with. Just search d3dx9 download. (If you need to)