Press Alt+F11 to open the VB Editor or go to tab "Developer" on the ribbon, click on "Visual Basic" button. Double click on "This Workbook", if you can't see it expand the list by clicking on the + sign.
Paste the following event code to the workbook module:
Private Sub Workbook_Open()
Go back to Excel. Save the workbook with file extension *.xlsm, this is important.
Close workbook and open it again. The workbook Book1.xlsx in folder c:\temp is now automatically opened. (If it exists..)