Hello,
After many test,
There aren't problem of rights with the Office 2000 and 2003.
Working with Open Office, there aren't problem too.
The problem was the strange change rights of files from Office 2007 alone.
Then, i think it's a bug from Office 2007 to Samba.
See you
well.. there are almost 2 solutions..
first of all, don't use office 2007.. it's the simplest one.. use office 2007 license and install office 2003 (license downgrade.. search info about it)
then.. update your server.. 7.0 is old, too old
then if problem still exists, it a M$ problem, not a Sme/linux one.. so ask M$ to solve it..

ok.. I'm flaming..

Regards
Stefano