automatically execute a macro in excel

hi,
how can i automatically run a macro, in other words the document must be opened without the enable/disable macro dialog. it has the code as mentioned below. i am totally new in excel, so i hope you guys/girls can help me.
thanks
anja

Private Sub Workbook_Open()
Dim Naam As String
Naam = Environ(“username”)
Range(“A1”).Value = Naam
End Sub