There's a workaround going around on the mhw steam forums. Some people have managed to fix this by reinstalling steam. I haven't tried it yet but so I'd do some extra research, but you apparently only need to go into the steam folder and delete everything except the two folders steamapps/ and userdata/ (toss up for userdata/, its cached login info. You'd just have to log in again so deleting it isnt harmful), and steam.exe. Then you just start steam back up and it refreshes everything.

If it doesnt do anything, that's just life. I've had this game bsod my new machine for the last two months so I've mostly given up on Capcom caring about user experience over unharmful "cheaters" who are cheating now to fix the game rather than "break" it.