I have installed the DNG codec on my laptop. Restarted, but afterwards couldn't see any thumbnails for my DNGs (DNGs created with the Adobe DNG convertor from an EOS 10D, and also DNGs created with CaptureOne 4, same camera).
On top of that, thumbnails of other file formats weren't shown anymore, either. Most of the time when browsing to a folder, the correct thumbnails blinked up for a moment, and then most of the images showed the generic image icon, while a few (mostly 2 or 3 out of a folder of 10-20 images) still showed the thumbnail.
After deinstalling the codec, everything went back to normal.
Oh, forgot to mention: The OS is Vista Business 32.