I have been having trouble in game with persistant creatures. For quite a while I've had in random areas, even interiors, where mammoths, and occassionally giants or netches, appearing out out nowhere when I enter a cell. I've even had that problem with a new game in the beginning waggon scene. Just today after placing a spider in a room and then deciding it didn't work and removing it, when I coc into that cell from the start menu it is still there. I've been using "Kill" and "Markfordelete" in game to get rid of them, but what is causing this and how can I fix it?

If I need to reinstall my game, what folders do I need to keep to preserve a mod in progress?