Naw, it wasn't any virus I could come up with (believe me, I checked!) I'll probably never know the truth, though, because I finally gave up and reformatted, which of course fixed everything. So, maybe it was a virus, I guess, but it's a non-issue now.
It was time to reformat anyway; I figure it had been at least two years since I last did that, and it's good to clean house once in awhile.