To filter merged cells in Excel 2003, you need to follow these steps:

1. Select the merged cells that you want to filter.
2. Click on the "Data" menu and select "Filter".
3. Click on the drop-down arrow in the header of the merged cells column.
4. Select "Text Filters" or "Number Filters", depending on the type of data in the merged cells.
5. Choose the filter criteria that you want to apply, such as "contains", "equals", or "greater than".
6. Enter the filter value in the text box.
7. Click on "OK" to apply the filter.

When you apply the filter, all the rows that contain the filtered data will be displayed, including the rows that are hidden by the merged cells. You can then sort or manipulate the filtered data as needed.

Note that filtering merged cells can be tricky, as Excel treats merged cells as a single cell, which can cause unexpected results. It's generally best to avoid merging cells if possible, and instead use other formatting options such as centering text or using borders to create a similar visual effect.