I have Excel 2003 with Windows XP.

From the File Explorer, every time I double click on a .XLS file, it
opens in a separate instance of Excel. This is a problem when
working on multiple workbooks, because linking and copying becomes
cumbersome.

What settings (File Explorer, Excel, or Registry) do I need to change
in order for all files to open in the same instance of Excel?

Thanks,
AABob