right with all my saves when I enter a building the game crashes to desktop.
if I try to fast travel it crashes to desktop.
this happens with all my saves both auto and manual saves.

why is this happening.
have my saves been corrupted
do I have to start the game again?

Try verifying your game cache in Steam. Go to the Steam menu, go to FNV and in the dropdown select properties, then do the 'verify game cache' thing...it may help.

tried that and it verified and corrected one file but still getting the crashing thing.

I'm getting this same issue and it is pissing me off. At first I thought it was a mod, but no. It's the game. The mods I have installed shouldn't affect anything that would cause the game to crash on save, as I've been running with the same damn mods since I started this playthrough about a week ago.

Followup. I just reloaded my previous savegame, returned to the same area where I had the savegame crashes and the game now saves properly. This definitely seems like a game problem.

hmm will try leaving the cell and waiting a few game days see what happens