Make a custom value list (File>Manage>ValueLists) with values of Yes and No.
Place the field on your layout, set it to the radio button form and check to use this value list.
Then have your field set up (File>Manage>Database) to autoenter "No" on creation.
In File / Manage / Database... select your field and click the Options button, and under the Auto-Enter tab click the "Data:" checkbox and enter the word "No" in the adjacent field (sans quotes of course). Thereafter, all new records created will have the value "No" in them, and as long as your radio button value is also "No", then it will be checked (and you will be able to manually change it to "Yes" if you want). Wall-Ah.
== EDIT ==
Ninja beat me to it, but I'll leave mine as it explains the Options thing.
Many thanks Ninja and Shudder - that works a treat.