It may be that you have one cell with a faulty formula reference and all
other cells with the #N/A error value are being caused by it (i.e. they are
either directly or indirectly pointing back to that cell).  You can use the
Formula Auditing toolbar to trace a cell that is the source of the problem.

To do this in Excel 2003, select View-Toolbars-Formula Auditing to pull up
the toolbar.  Place the cursor on any one of your #N/A cells and click the
Trace Precedents icon in the toolbar. It will display an arrow pointing to
the cell that is causing #N/A in the current cell.  Click on the cell that
the arrow is pointing to and select Trace Precedents again to find the next
earlier cell in the chain.  Continue this procedure to trace back to a
source cell (you'll know you've reached it when the Trace Precedents
procedure fails to point to another cell and displays a message).