For an unknown reason, Adobe has seen fit to remove the ability to view PSD files as thumbnails in Windows Explorer. You may not have noticed this if you upgraded your version of Photoshop from a previous version, as the ability to display PSD files as thumbnails is driven by a DLL file that was distributed previously, but is now strangely absent.

Luckily, Mike Golding noticed that you can download the needed DLL from DLL Dump, and by simply dropping it into the correct folder, resurrect your PSD thumbnails in Windows Explorer.

Now, I don't know who actually owns that DLL, or whether DLL Dump has the right to be offering it. But if you're curious, there it is.