It doesn't appear this has been resolved yet, so let me revive this thread.
I'm seeing this same problem with tif files in both Vista 32 and 64 bit versions, especially in folders with these files that have never been viewed in a thumbnail preview (icon view) mode before. I'm seeing the same behavior in newly viewed folders that contain images in other common graphic formats like jpg, gif, and pngs. Cpt files however will preview fine in Vista's Windows Explorer.
This is occuring when using the Organize/Layout/Preview Pane under any of the icon views in Windows Explorer. I can see that a thumbnail preview is attempted sometimes in folders with a large number of images, but the screen quickly redraws and replaces the thumbnails with the Corel X4 photopaint red camera icon. It seems like some caching of older thumbnails may be coming into play here.
This problem does not occur if Vista is told the default program associated with the file type is Corel Photopaint X3, rather than X4. Apparently, some file associations previously associated with another program MUST be changed via Vista's default program tool. Photopaint's Tools>Options>Global Tools>Filters>Associate method does not always make the expected changes when working under Vista. This is very different behavior than the way things had worked under Windows XP.
I've noticed one thing that I believe is of particular note when troubleshooting this problem. Windows Explorer keeps thumbnails in cache somewhere on the hard disks. Folders where filetypes associated with Photopaint X4 have previously displayed as previews will continue to do so, while newly created folders containing files with the same filetype will not.
One may clear the thumbnail cache on any particular hard drive by:
Right click on the hard drive icon of choice
Selecting properties
Under the general tab, pick disk cleanup
Check the thumbnail box (uncheck the other boxes unless you want to clear out those items as well)
Select the OK button
If you return to an icon folder view after clearing the thumbnail cache, you'll see the default X4 icon and not the thumbnail preview on non Corel graphic file types that were associated with Photopaint X4.