Hi WickedShark,
Here is one way you can do this:
1) hide the text box for comments if the user selects anything other than no (right click on text box field and select "conditional formatting". You can then hide the control using a test on the value of your drop down list box value)
2) make sure that the value of the the text box cannot be left blank by the user: right click on text box field, choose text box properties and under "Validation & Rules" choose "cannot be blank").
4) Make a rule on the drop down list box to set the value of the text box comments depending upon the selection. If the user selects yes/na you will need to add some filler text into that text box so that the user won't be prompted to enter text since the text box field can't be submitted with a blank value (I suggest putting some text in there that will indicate what selection the user chose so you know this message is from the form and not something the user put in there). Also it would be a good idea to add a rule that when the user selects "No" that the value of the text box for comments gets cleared out (fileds value = "") This will help with your overall validation of that field.
Make sense? Post back if this is not clear enough, it's a pretty simple process.