Free SVG Viewer

Adobe has a free SVG viewer here :
http://www.adobe.com/svg/viewer/install/
They've apparently stopped supporting it, but you can still see the download link at the bottom of the page for WinXP.

For thumbnails from Windows Explorer, I downloaded Renesis Player from here :
http://www.examotion.com/index.php?id=product_player_download
You may have to restart Windows Explorer (or even Windows) to get SVG thumbnails generated.