No matter which of the suggested formulas you use, you'll be better off to
include a cell reference in those formulas:
=cell("filename",a1)
If you fail to include a cell reference, then when excel recalculates, the
formula will evaluate using whatever worksheet/workbook is active.
(You can see this by using two workbooks, arranging the windows (window|arrange)
and putting:
=cell("filename")
in each window.
Then look at the opposite window.