Thumbnails - XPUnder the Files and Folder section you will see a checkbox next to the phrase: Do not cache thumbnails. If this box is checked there will be no thumbnails, so if you want thumbnails you will want to check it. Click Okay when that box is checked.
To start seeing thumbnails you have to change views in Windows Explorer. On the menu bar click View | Thumbnails and Windows will replace the normal file icons with thumbnails of the images. Windows will also create a new file in that folder called Thumbs.db where the thumbnail information is stored.
The Thumbs.db file will be created in every folder you designate to display thumbnails. To apply your changes to all folders while in Windows Explorer, navigate the menu to Tools | Folder Options and click the View tab. From there make your changes and then click the Apply to All Folders button before you click the Okay button.