Recently my friend has got some problems viewing thumbnails in his folder. It only show the icons of the bmp, jpeg ,gif and png instead of the thumbnail of the picture. This happen after he try to remove some viruses and malwares. The registry may have been modified and that is why you are not able to view your thumbnails.

You may go to view your registry by typing the following command at the Start -> Run
regedit

It will open the registry editor. Do not try to delete or modify anything as it may affect your operating system. Click to open the HKEY_CLASSES_ROOT -> .bmp -> OpenWithList. If you do not see the ois.exe and PictureViewer.exe, it means you will not be able to view the thumbnails.
Close the Registry Editor and perform the below command to rectify it.

Type the following command at the Start -> Run:
regsvr32 /i shimgvw.dll

Once you see the message "DllRefisterServer and DllInstall in shimgvw.dll succeeded", it means you can now view thumbnails in your folder.