I recently started have the SAME problem. Windows Photo Viewer had been working swimmingly over the last few months on the same exact images without any problems.
I suspect Microsoft updated some code somewhere that got pushed out as an update -- that's now causing the problem.
As a diagnostic, I tried right-click "Open with..." and chose Paint. The same file opened without any problems.
Thumbnails are all being generated for ALL images without problem.
Yet Windows Photo Viewer cannot view ANY of my image files. Paint has no problems.
Similarly, right-click "Open with..." and chosing Microsoft Office 2010. Image loads with no problems.
If the original poster has the same success using other programs -- then the problem is with Windows Photo Viewer -- Microsoft's not having it recognize the code permissions or something.
This is on Windows 7 Professional 64-bit. Most recent updates from Microsoft applied.