Your information is misguided. shmedia.dll is not a core DLL file found in Windows 7. It is a Windows XP-era DLL file that is no longer used in Windows 7. What you need to do:
Ensure your system is not low on hard drive space. This will automatically disable the feature.
Ensure no new programs have been installed, such as QuickTime, etc. These may actually change the file associations and ruin your thumbnails. To get them back go to Start -> Default Programs -> Set Program Access and Computer Defaults -> Microsoft
Right click "Computer"
Choose Advanced System Settings and find System Properties
Click on Performance and select Settings
Ensure "Show thumbnails instead of icons" is checked.
If this still does not work you are dealing with either a codec problem as a result of the installation of 3rd party software, or a problem with the registry