change default checked button from "Gimme a blog" to "Just a username" on signup


I would like new users (invited via signup code) to be given the choice of whether to create a blog or just to have a username but I would prefer the default pre-checked option on the signup page to be "Just a username, please" rather than "Gimme a blog!". This would avoid users inadvertently creating a blog when all they need/want is a username but alllow those who want the option of creating a blog.

I'm sure this must be fairly straight forward but can't work out where/how . .