I go along with those things mentioned as causes but add "User sticky fingers" as a very common cause to the list
.
Comment on Randy's case is as I said before, somehow in converting from Windows 7 to Windows 8 he has ended up with a User account named Windows 7. Now whether this actually appears as a real Account in his Windows 8 installation or not, and assuming he did not create a User named Windows 7 oneself then he has done some thing wrong - perhaps along the lines I suggested earlier. One thing is for sure, if he didn't create it and he has now reduced its User folder security permissions other problems of some kind or another arising downstream in time seem to be of pretty high probablility.
I also go along with theducks, giving full control to Everyone in ones User folder is just a shotgun attack asking for trouble downstream in time. Everyone should not have any permissions whatsoever.