In Excel 2003, go to Tools -> Options -> General tab.
Make sure the option, ‘Ignore other applications’ is checked. Now all the Excel files will be opened as separate instance and you can move the Excel files individually across the monitors.
In Excel 2007, Click the Office button -> Excel Options -> Advanced.
Under General, check ‘Ignore other applications that use Dynamic Data Exchange’.
Open the file by double click, and now it would open in new instance.