Please ignore this post I worked it out myself and fixed it

There's a hidden file called thumbs.db in every folder that stores the thumbnails. You can see that file and delete it in programs like ACDSee (for example) or in Windows by selecting Tools / Folder Options and unchecking Hide Protected System Files. If you delete it, Windows will regenerate it when you view the folder as thumbnails.

You should be able to accomplish the same thing by copying all the files to a new folder.