I too have had this problem occur. It's only begun to happen today, I am almost positive it has to do with me doing a disk cleanup and deleting Thumbnails (says windows automatically keeps a copy of all the thumbnails, if you delete them it will recreate as needed. I figured this was okay to delete, as from what it said I assume that it simply shows thumbnails that you try to view, and any that are part of old files that I don't view anymore will remain deleted and save some space).
So, should I bother taking any steps towards fixing this, or will it eventually occur less and less as I view more of the files that I have deleted the thumbnails for? If so, than would it possibly help to highlight all the pictures I have (only things it's occurred for so far) and open all at once, so that it loads all the thumbnails? I'm not entirely sure how it works, someone please let me know if it will either eventually fix itself, or if I should do a system restore or use another method of fixing (system restore won't be much of a problem as I made a sys restore point last night, but if I can just leave it as is and it will fix itself as files are viewed, than that's best)