Excel corrupts files?

Good day, dear forum users.

Users logon the server. Thay make to the Excel-documents changes, save and exit. These documents may contain graphs, formulas, or normal data.
Recently I found out that some of these files when you open Excel identifies as corrupt and offers to restore, but could not recover. The result - a lot of lost information, time.
I even don `t know what could be the reason. Maybe someone has already faced similar problems - please share how to solve this problem and what is wrong.

Tech. data: I'm using Excel 2007, Windows 2003 Server (Windows 2008 Server). And one more thing - users run the Excel-files from a specially written program, not from Windows Explorer. But the principle of launch excel files from this program, as I understand it, is no different from the launch from Windows Explorer. So I think that is not the problem. Please, help me.

P.S. sorry for my English, I'm not english-speaking user .