Victory!
ilovejedd, I'd taken all those steps, but it turns out there are some counterintuitive aspects to sharing from Vista (at least to XP). First, right click and "share" is irrelevant; second, Properties>Share will be grayed out, you have to click Advanced Sharing; once you've set the it to share with Everyone, YOU'RE STILL NOT DONE; in the Security tab, Edit and manually add Everyone.
I'm still not sure how to get the drives to share, but I'll leave that for another day (it would only be proof of concept anyway--not sure when I'll ever need it).
Thanks for prodding me to attack the problem again