I didn't see this posted anywhere so I thought I'd share:
My Simplify 3d 4.1.2 continually crashed immediately after starting. It showed I had an STL file already imported, even after repeatedly uninstalling and installing the software. I tried Ccleaner on the registry, restarts, and cold starts, all to no avail. I finally realized it was loading the STL file from my user profile. I went in to the AppData/Local folder on my user profile and deleted the Simplify 3D folder and this worked. I probably could've just deleted the offending STL file, but at that point I was pretty well frustrated. I believe the STL file somehow became corrupted on my drive, and loading it caused the crash. Hope this helps anyone who ever has the same problem.