When you say "cannot open it" do you mean..
Can't save or open the attachment in there email client, or once saved to disk they can't open it
the reasons for both are completly different.
If its can't save or open in email client this can be solved in (assuming Outlook Express) tools> options> security tab & uncheck don't allow attacthments.
If its can't open once saved to disk, does anything give a error message ?. or if using XP right click on file if its blocked unblock it