We would like to add a horizontal line to the header of an Excel file to be repeated on every page.

To add a horizontal-line we first need to create an image of a horizontal line in your favorite imaging software.

Create a PNG image 660 by 3 pixels and color it in some shade of gray.  Save the image with an appropriate meaningful name.  You will be using this file each time you need to add a line to Excel header of footer.
Now, insert this image into Excel header:

Note that &[Picture] code should be bellow any other code that you have in the header.

Now we have a nice Excel header with a horizontal line that serves as a visual separator:

The same technique applies to the Excel footer.  Here &[Picture] code should be above any other code that you have in the header:

There is one more method to place a horizontal line in Excel 2007 header, without using images.  Just with underscore symbols.  If you place this into your custom Excel header:

You will get these results bellow:

This is not bad, for a simple low-tech solution.