file is locked by other user

[Camuffo]

Hi all,
i've got this problem:
there are John, Mary and Paul with Microsoft Office 2007 SBS.
John open test.xls, Mary try to open test.xls and she retieve this message:
File text.xls is locked by "Paul".
Paul don't have opened test.xls but John still open the document.

I've tryed to search on google but seems the nobody have my problem.
Test.xls in on a Server Windows 2003 STD and there is an AD.
Please advice!
Many thanks