As you can see on Vista 64 the 32 bit windows explorer the MobiHandler works as it should. The mobi tab in file properties works as well.
In the 64 version of windows explorer the only thing that works is the Titles and Authors columns showing the books info. No Thumbnails and no Mobi tab in file properties window.
I tried already clearing the thumbnail cache it does nothing.
I don't have UAC enabled as it's annoying but Avira doesn't like the setup still but once I temporary disable it the handler installs fine.
Edit: I've been playing around with it and I have found once I let the thumbnails generate in 32bit Explorer that they show up on the 64 version but they wont generate any new ones added you have to go to the 32 bit version for it to generate them then the 64 bit will show them. I don't know if that helps figure things out or not but it's sorta a work around. No Mobi tab though in 64 bit no matter what I try.
Edit again: I just noticed that is does not work for non-drm mobi files at all in either 32bit or 64bit. No Thumbnails or Mobi tab just Author and Titles columns working. The thumbnails and mobi tab it works on are AZW only on my pc.