Oh dear. The thumbnail handler has never worked on x86 systems because it is dynamically linked to "zlib1.dll", a library that Blender ships as "zlib.dll". This update, which needs to be unzipped to both windows libs folders, addresses the issue by linking it to Blender's own Zlib build. I've been able to confirm for myself that the handler now works correctly on Vista x86.

The x64 build is statically linked and was not affected.