Since you people are no help, I will log my own attempts to resolve this issue, in case I need to do it again in the future.
I use Foxit Reader for PDF viewing, because it's much better. I installed Adobe Reader 8.1.1 to get my PDF thumbnails in Windows Explorer back. I also have Photoshop CS4 installed. Here are my steps so far:
1. Ensure that my folders are in "Large Icons" view, make sure "Always show icons..." option is disabled.
2. Uninstall Foxit Reader & Adobe Reader.
3. Remove Adobe Reader & PDF file type entries from registry.
4. Ran Unassoc to remove the PDF file type from Windows.
5. Ran Disk Cleanup to remove Thumbnail files.
6. Installed Adobe Reader 8.1.2.
* At this point, I effectively had a clean install of Adobe Reader. However, my PDF file thumbnails still show up as the stupid Adobe icon.
7. Ran xplorer2 32-bit in Thumbnail view. My PDF thumbnails show up both in the file explorer view and in the preview pane (Draft and Native).
8. Ran Adobe Bridge CS4. PDF thumbnails show up there, too. So, at least I now have 2 effective workarounds to Adobe Reader's ridiculous shortcoming.
9. Ran the 32-bit Windows Explorer. No problem with PDF thumbnails. Hot damn!
10. Closed 32-bit Windows Explorer. Opened regular Windows Explorer and browsed to the same directory. PDF thumbnails still working.
11. Browsed to another folder with Windows Explorer. No PDF thumbnails.
12. Reinstalled Foxit Reader 3.0 so I don't have to suffer through actually using Adobe Reader. Made sure not to set it as the default PDF reader.
13. Verified thumbnails in 32-bit Explorer still work. They do. *******.
There you have it. 3 separate workarounds. Maybe Adobe will fix this problem in their next release. Maybe they'll also create a PDF reader that isn't 100mb. Don't hold your breath. Thank me and follow my instructions instead.
Here's a proper fix: http://www.pretentiousname.com/adobe_pdf_x64_fix/index.html
(Running a 32-bit program to trigger the thumbnail generation only means you'll get thumbnails for the files you view at the time, since it puts them into the icon cache that's shared by 64-bit processes. The fix linked above removes the need to do that at all and allows 64-bit programs to generate PDF thumbnails by themselves.)
I am also running Vista 64-bit. I always used the 32-bit Explorer in stead of the 64-bit to be able to see thumbnails in explorer windows. Until about april / may 2011 ( 04-2011 05-2011 2011 WOW windows on windows ) this worked for me. I had some updates, both Windows and Adobe. And now new files don't get a thumbnail anymore, my pdf files wich had one will loose theirs when I open a window.
The fix mentionned above (Pretentiousname) didn't work for me.
If thumbs aren't working for 32-bit apps, the fix won't fix them for 64-bit ones. All my fix does is act as a bridge betwen the 64-bit and 32-bit worlds, but if things aren't working in the 32-bit world then it'll be a bridge to nowhere.
Doing a repair or re-install of Adobe Reader usually fixes things, though. Well, as much as anything is ever fixed in the wacky world of Adobe's PDF software.
That is a clear answer. Thanks.
I did a reinstall, no luck, it probably has to do with the fact that my Acrobat Pro is still 9.0 and the reader is upgraded to 10 or X (maybe it X-ed my thumbs).
I tried the following simple solution, which in my case worked:
In Explorer I had to addres the pdf with the "open with" command to be opened by "Adobe Reader X".
All thumbnails started appearing again in 32-bit Explorer.