Long time later, but just for completeness.
This problem (registry and Group Policy both being ignored - thumbs cached regardless) still exists in Windows 8.
The solution of denying access to the .user\myname\appdata\....\explorer folder to all users and leaving it read-only for myself still works however.
Since the W8 system is completely new and the ghost (negative) thumbs mentioned are still around, "evolved"'s explanation of thumbs embedded in the metadata must be correct. W8 cannot possibly know that the image originated as a negative scan, so it must be finding it with the image.
Still a bit of a puzzle though.
W8 no longer has thumb-size slider in its ribbon so the positive/negative flip/flop doesn't occur, but no matter which thumbnail size is chosen, a negative thumb appears, indicating that W8 Explorer must be resizing the embedded metadata thumb, not the final image.
Here's a very useful website Jeffrey's Exif viewer
which proves that the thumbnail is embedded in the metadata