Seems the new launcher is giving an "unknown error" for some people.
Screenshot: http://imgur.com/1vxuKhg
Running java -jar minecraft.exe from command prompt returns: Error: Invalid or corrupt jarfile minecraft.exe
Running the old minecraft.exe or the .jar file works just fine.
Hotfix by Nvidia, available here: http://nvidia.custhelp.com/app/answers/detail/a_id/4378/