In Microsoft Windows XP, you can select the Thumbnails view for any folder, thereby gaining a quick way to find images. In some earlier Windows versions, you first had to right-click on the folder, choose Properties, and check a box titled Enable Thumbnail View. Either way, Windows generates a small thumbnail image for every file in the folder. For many file types, the thumbnail is just an over sized icon, but for image files, it's a small version of the image. Because it takes time to generate the thumbnail images, Windows caches them in a hidden file named Thumbs.db.
In theory, the Thumbs.db file stays up to date with the folder's current contents, updating thumbnail images as necessary. In practice, the file can sometimes get out of sync, and may also wind up containing thumbnails for files that have been deleted from the folder. You can right-click on an individual file and choose Refresh Thumbnail, but for many photos in many folders that's too slow. Since Windows will re-create the thumbnails as needed, an occasional general cleanup is a good idea, and it's easy to do.
Select 'Search' from the Start menu, choose to search for files or folders, and search your local hard drives and their sub-folders for files named 'Thumbs.db'. When the search is finished, you'll have a list of all the thumbnail files. Click within the results area, press 'Ctrl+A' to select all of them, and press the 'Delete' key to delete them. Click on the 'Yes to All' button. Now all of your folders with pictures will get new, fresh thumbnails the next time you view them. In addition, depending on how many such files you had, you may save quite a bit of disk space.