Copying cells between worksheets

Hello - I need help resolving an Excel 2007 workbook and worksheet error!!! It is a shared workbook between myself and one other user. When either of us opens the workbook from a file or from an email attachment, and one of us inputs something into a single cell on a single worksheet...that info is copied over other worksheets (even though they are not selected worksheet tabs!) and it is deleting the content on that same cell on the other worksheet tab(s)!!! How can we resolve this??? We need an answer today before 4pm ET. Thanks!

Thank you!!! Problem resolved!!! And to think my IT help couldn't figure it out and Google searches and Excel ? searches couldn't tell me how! So glad I just joined this Forum!