Excel opens new instances when I text-open files

[Eric]

I often open tab-delimited text files in Excel 2003/Windows XP.

I would like the files to open in the instance of Excel I already have open,
but it always opens in a new instance. (when I open *.xls files, it will
open in the existing instance of Excel)

I have set the file extensions to open automatically when I double-click
them, but the same thing happens if I right-click and select "Open
With/Excel".

Is there a setting I can change so that text files will open into the
current instance of Excel? Note that I do not have the "Windows in the task
bar" option selected.

Thanks,

Eric