I have a very strange problem on excel 2003 (11.6355.6360) SP1

When I create any formula (eg: SUM(A1:A10)) in a new document, it
works. If I change values in A1 -> A10 it is update in real time.

If I save the document, exit Excel, then reload the document (thus
reloading Excel), the formula doesn't work anymore : the result is
stuck to the last value when I saved the document.
If I retype the formula, then it returns 0 whatever values are in A1 ->
A10.
If I select the cells A1->A10 with the mouse, I got the correct sum in
the status bar....
In tools->option, automatic compute is checked, and if I click on
compute now or compute document, it doesn't work.

If I save a document with no formulas in it, the reload it, I can
create formulas that work into it.
If I save a document with at least one formula in it, then reload it,
the formulas don't work anymore, and any new formula I create in it
doesn't work either...

If I send the document to a firend of mine who loads it into his excel,
it works !!!! so the problem is in Excel , not in the document.

I tried to repair office with no result.

HEEEEELLLLLPPPPP !!!!!

Information complement :

If I save an empty document, then the first document I load just after works
fine.
If I uninstall and reinstall Excel, the first document I load just after
works fine.

I found no viruses or spyware on my computer...

Very strange....

Any idea ??