To put a horizontal line in the middle of a cell in Excel 2003, you can follow these steps:

1. Select the cell where you want to add the line.
2. Click on the Borders button in the toolbar (it looks like a square with lines on it).
3. In the Style section of the Format Cells dialog box, select the type of line you want to use (e.g. solid, dashed, dotted).
4. In the Color section, choose the color you want for the line.
5. In the Presets section, click on the Outline button to remove the border from the cell.
6. In the Preview section, click on the line that represents the bottom border of the cell.
7. In the Border section, select Inside to add the line inside the cell.
8. Click OK to apply the changes.

The line should now appear in the middle of the cell, separating the numbers above and below it. You can adjust the thickness and style of the line by going back to the Borders button and selecting More Borders from the dropdown menu.