There may be a cleaner method, but this seems to achieve your goal.
1) Not sure where it is on 2003, but you want to find 'New Window'. This will make a duplicate of the spreadsheet you are working on and ensure any changes to one will appear in the other window
2) If you are in 'full screen' mode of excel, you need to get into the mode where each workbook is its own window. Usually, this is achieved by hitting the 'Restore Window' button on the top left of the workbook window (Usually just below the 'X" close Excel button on the top right of the Excel window).
3) Now you can position the windows however you see fit.