There are a lot of people asking about this, and a great many sites which tell you how to set it up. They all entail copying DLL files and registering them, etc. Sadly, these methods do not work on 64-bit Windows Vista/Windows 7. Here is what works:
- Ardfry Imaging‘s PSD codec for Adobe Photoshop Documents
- MysticThumbs Thumbnail provider for Windows 7, Vista and XP
They both cost money ($20 and $25, as of this moment).