I had a problem where the launcher would crash both before and after the game ran. I would click 'PLAY' to bring up the Launcher window
which then allows you to change Options and click PLAY again to launch the game itself.

After the 1.2 update the launcher would crash every time I ran it as many as 6 times before it would run normally and allow me to run the game.
Also after exiting the game the launcher window would be still be onscreen and would crash.

I stumbled on the solution while trying to fix another game - Left 4 Dead 2 - which has recently been having poor network performance.
I found some people mentioning that a recent driver installed through the Windows Update process for my onboard Realtek LAN was poor
and so I rolled back the driver from the October 2010 to one dated 2009 and it definitely improved L4D2 but as a by-product FNV now starts
and exits properly - the launcher hasn't crashed once since I made the change.

The NIC driver update was in October and the 1.2 patch was November and the issue only arose after the 1.2 patch so it wasnt obvious to look
at the driver and the general rule is that the latest drivers are best but NOT in this instance.