Scott Hanselman wrote another article about thumbnails disappearing from Windows Explorer under Vista. I had this same problem happen to me a short while ago, and ended up giving up on the solution, since I was going to reformat my system in a few days anyhow. Kinda lame that Windows doesn’t automatically repair itself when this happens, but anyways…

He’s offering a registry patch to try and fix the problem. It appears to boil down to a ShellEx registry key value, as well as potentially a PerceivedTypes registry key, found under each file type’s extension.

He recommends downloading the CCCP (Combined Community Codec Pack) to handle almost all video/audio codecs you’d ever stumble across. I’m usually apprehensive about codec packs, since they tend to be overzealous and do more harm than good, but the CCCP seems well-rounded and thought through.