If they are set to make local copy to edit, you could have issues like that.

Dazza13 wrote:

We have a workbook in 97-2003 format, the problem is that 2 users are able to
open and make changes to the workbook even though it is not shared out.

What we require is that if one person opens it then the second will get the
message that it is open and to open it as read only but this is not happening.

One user is using excel 2003 and the other is 2007 I don't know if this
makes any differnece. I have tried saving as html and the saving back as a
workbook but this has had no effect.