Use the script step 'set field' to set the value or you can use an Auto-Enter Calculation that set the value when a new record is created.
Or just enter that value into the "data" box on the auto-enter tab.
Interesting note: Because of formatting issues each line (for example "illness Injury within last five years") is made of two separate check boxes, one no and one yes. Originally each field is set as text.
I like the sound of your ideas but Im not sure of how to preform a calculation that yields yes or no as a result(or really any result that will just get me that little x. ANd its the same sort of problem for the "auto enter data box, on the auto enter tab" what value will just get me an x in the box that later I can change without worrying about it reverting in the future?
If your check box values are defined as "yes" and "no" in the value list and you want "yes" as the default value, enter Yes (no quotes) into the data box.
You might find it informative to make a second copy of this field and format it as an edit box, then click check boxes in the check box formatted copy. You'll see the actual data that gets entered in the edit box copy.