scfr, if it's just Turbo Tax information, I think a hammer would be plenty. Or you could even surgically dissect one for educational fun!
I hate disagreeing with Tina (because, you know, hell hath no fury like a woman scorned), but I would not advise microwaving just as I would not advising pitching it into an open fire... or acid baths for that matter. It's just that metallic objects in a microwave constitutes a fire hazard, no matter how "safe" you make it. Some computer components may also produce toxic fumes during chemical reactions, but I don't know if hard drive platters are one of them. Still, I just don't think there's any reason to go that way.
As for your last question, even bits of hard drive platter can be read, but again, we're talking about very expensive equipment used by trained technicians sent in by very determined and well-funded identity thieves just for tax info. You're not... evading millions from the IRS are ya?
Because, seriously, there's nothing to worry about.
Honestly? Hard drive + Hammer = Good times.
(Although I also think it's kind of a waste to smash a perfectly good hard drive, but... that's just me.)