There is a problem with the thumbnail extension dll. When ArtRage is updated the thumbnail dll registration gets broken.
You can re-download and re-install the thumbnail view application from here:
I'll be doing an update to the thumbnail viewer later this year to make it happier with Vista, and to fix a couple of other issues with it. But the one on the download page should work for you.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"