I tried these free versions as psicon.dll and psdcodec.dll, they did not work for Windows 7 64bit, even with the suggested fixes found on the Internet.
Finally, I purchased the codec pack from FastPictureViewer.com (link is in the answer above). For me it cost $14.95. The site is registered with Microsoft and this codec pack provides thumbnail view of PhotoShop ".psd" extension as well as support for many other image formats not normally supported in the Windows 7 OS. Apparently 48 format extensions. That is pretty good.
I installed this codec pack, works great. This codec pack provides support for Windows Photo Viewer to display the ".psd" extension, so I image that the remaining 47 more extensions will also display in Windows Photo Viewer. In fact, read the webpage at the codec site as it offers a lot more than just thumbnail viewing in Explorer.
My payment went through PayPal, so security of my payment was assured.
I used CCleaner (free windows file cleaner) and Comodo System Cleaner to remove the faulty registry entries caused by the free codec versions.
Sometimes doing it free is not always the best course to take and for what little it cost, Fast Picture Viewer provided the better option without headaches and endless Google searches.
Lastly, this is not a sponsored post for Fast Picture Viewer, nor am I affiliated with the site or its company. I thought I would confirm to viewers that this codec pack is a good one worth the money.