A few things are required to get media thumbnails (e.g. flv, mkv, ogm) working in Windows 7.
1. Shell Extension
Register the thumbnailing shell extension for every filetype you want to generate thumbnails for, by making a new .reg file with the following contents: (This shell extension comes with Windows Media Player 12, so make sure that’s installed.)
Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\.mkv\ShellEx\{BB2E617C-0920-11D1-9A0B-00C04FC2D6C1}]
@="{c5a40261-cd64-4ccf-84cb-c394da41d590}"
Run it. Then, rinse and repeat with .flv, etc in the place of .mkv above.
2. Filters
You must have a complete DirectShow filter chain avaliable for displaying the required file formats. 64-bit Windows requires a 64-bit filter chain. Get the standalone filters from the MPC-HC project, regsvr32 the FLVSplitter.ax and MKVSplitter.ax, and install latest ffdshow-tryout.
3. Filter Priorities
Thumbnails for other formats don’t work with the Microsoft DirectShow decoders that come with windows, so use Win7DSFilterTweaker to set ffdshow to preferred over microsoft decoders for all video formats (except VC-1 i guess).
4. Apply
Restart. Killing the explorer process might be sufficient. If there’s still trouble, you can reset the thumbnail cache on Windows 7 under Accessories > System Tools > Disk Cleanup > (C:).