I meant to post earlier that I was able to resolve my issues regarding editing of HDV video. I still don't know why the problem occurred, but I have a fix. After several calls and chat sessions with the Windows Vista SP1 Support group I found that loading the FFDShow codec fixed the problem. I can again see my HDV video (in dvr-ms format) and can edit it in Movie Maker.
Something about the steps still leaves a weird taste in my mouth because this did work before SP1, and the MPEG-2 codec and corresponding filters should have worked after the SP1 upgrade. Why I had to load some 3rd-party codecs for this scenario is still a little puzzling.
If I get a chance, I might restore the image from my Windows Home Server just before the SP1 upgrade and try it again... or I might just leave it alone. If it ain't broke, don't fix it... right?