It appears that Windows 7 has an even more limited ability to view thumbnail images than Windows Vista. The old (nVidia) DDS and (Thumbplug) TGA plug-ins don't work (even viewing the thumbs for PDF files don't work!). If you're looking for a solution, don't waste time trying the different registry hacks, file associations or what have you as none of them provide a one-stop-fix to the problem (none of the ones I tried worked).
As an alternative solution MediaFile Assistant (mfa101.exe)
still works, although it's a bit clunky now (it's not been updated for a whilst).