Ariston
Has it always done that or just a recent development?
How was Office/Excel installed?
When installing you have the option to select "run from my computer" as
opposed to "run from disk".
I would check those settings first through Contol Panel>Add or Remove.
Scroll down to Office and "Change/Remove". Pick Change and then "Add or
remove features" then work your way through the steps.
If answer to question above was just a recent development try a "detect and
repair" through Excel Help.
Gord Dibben Excel MVP