Update to the aforementioned:
After more headaches, it occurred to me that the most recent addition to my computer's bitter and twisted little days has been a dodgy upgrade to Windows Media Player 9. I did this in the mistaken assumption that upgrading was oftentimes synonymous enough with "improving" to be worthwhile. Plus it managed playlists, and could at least rip a song or two. But, you know, it was really the most precarious piece of the puzzle. So, I got rid of it, and I think my problems are going away. Even so, I'm sure I'm still going to squint suspiciously at my computer at work, expecting (as always) that it will double cross me again.
Testing out the "avoid unecessary Microsoft entities" theory out later, I uninstalled the last remnants of "Microsoft Intellipoint" on my iBook (Intellipoint came with my optical mouse) and now the mouse works properly for the first time since I bought it. Huh.
There's no moral to this story, really. I just like it when stuff works.