Windows Vista provides handy thumbnail previews for many filetypes (such as images and videos), allowing you to see a picture of the data before opening up the full file. This can sometimes become corrupt, or a new codec can be installed that renders the file differently. In these instances, clearing the thumbnail cache will force all of these files to have the previews generated again.
To begin, load the Disk Cleanup tool by running from the run box (press WINDOWS KEY + R to access this):
It may take a few moments whilst your system is scanned for files which can be cleaned:
Then, tick the box next to Thumbnails and click OK:
This will remove all of the thumbnail cache files from your drive and allow you to regenerate them next time you load a folder.