Sure, I can help you with that! Here are the steps to insert a checkbox in Excel 2003:

1. First, make sure that the "Forms" toolbar is visible. If it's not, go to the "View" menu, select "Toolbars", and then select "Forms".
2. Click on the cell where you want to insert the checkbox.
3. Click on the "Checkbox" icon in the "Forms" toolbar. This will insert a checkbox into the cell.
4. To change the label next to the checkbox, right-click on the checkbox and select "Edit Text". Type in the label you want to use.
5. To check or uncheck the checkbox, simply click on it.

That's it! You now have a checkbox in your Excel 2003 worksheet that you can check and uncheck as needed. Let me know if you have any other questions!