It is being used by another person or program

[adqo]

We have an Excel file that is being updated/edited alternately by Excel 2003
and Excel 2007, however never at the same time. The Excel file is not shared.
At some point:
1) Excel 2007 cannot save the file, error:
... .xls is currently in use. Try again later.
2) Vista cannot delete the file, error:
... The action can't be completed because the file is open in another program
3) Excel 2003 can STIL save changes.
4) Windows XP cannot delete the file, error:
... Cannot delete ..: It is being used by another person or program.
We cannot find a person or program which is using the Excel file. After a
reboot of the file-server the Excel-file came be removed.

Does anyone know what causes this problem?

Does anyone know how the Excel-file can be unlocked without a reboot of the
file-server?

Hoop someone knows,
adqo