After installing Acrobat from CS5.5 on windows 7 x64 generating pdf thumbnails in windows explorer got broken, here's how i fixed that.
1. open reader x preferences (crtl + k), go to "General" tab, click "Select Default PDF handler" button and choose Reader
2. if thumbnails got fixed after that - great!
3. if not or even got worse (for me Reader assigned wrong program to handle PDF files) go to Control Panel in windows, click "Default Programs" or something like that (i can't tell how exactly it is in english because i use polish version of windows)
4. in window that opens click this
5. browse for .pdf and click "Change Program"
6. in window that opens select Adobe Reader, click OK, close control panel window
7. now open Adobe Reader from start menu/shortcut, press crtl+O (open file) and navigate to any folder containing PDF files
8. you should see windows generating thumbnails in the "Open File" dialog
9. it's (hopefully - at least for me it worked fine) fixed
hope this helps