Hi Janice,

You can use Data Validation on the drop-down list to implement the required
functionality.

Try this:
- Double-click on the drop-down to open its properties
- Click on the [Data Validation...] button
- In the Data Validation dialog box, click on the [Add...] button
- In the second Data Validation dialog box, select the field on your
InfoPath form that represents the checkbox in the first drop-down list.
- Leave "is equal to" selected in the second drop-down list
- Select "TRUE" in the third dropdown list
- Click on the [Add >>] button
- Leave the selected field in the first drop-down list on the second row
intact
- Select "is blank" in the second drop-down list on the second row
- Type in a short description, e.g. "You must select an option from the
drop-down", in the "ScreenTip" field
- Type in an error message in the "Message" field
- Click on the [OK] button on all open dialog boxes to close them.

Does this help?

Regards,
S.Y.M. Wong-A-Ton