When you download 3D models from certified suppliers - such as SKF bearings as one example - you end up with a file that has the .swb extension. In order to create the part in SW, you must 'Run' a Macro to create the part.
Not always, but MOST of the time if you do this while you have a SW file open, SW will crash on you - so make sure you have saved all your files first before you try this. The best way is to close all your files then do it. Even at that, SW will very often crash. Once you reopen SW it works ok, but run the Macro before you open any files.
PROGRAMMERS - this problem needs to be fixed in SW asap.