How can I turn off gridlines in excel using VBA, without using ActiveWindow

If you have a reference to the workbook, you can just iterate over all of the Windows in its collection. If the application isn't visible, you should only get 1 but it's safer than trying to hard code an index:

Private Sub ToggleGridLines(target As Workbook)
    Dim wnd As Window
    For Each wnd In target.Windows
        wnd.DisplayGridlines = False
    Next
End Sub

Note that this will set change the display on the active worksheet in the workbook - why this is a property of the window and not the worksheet is beyond me.

EDIT:

Thanks to the link that @Tim shared, I realized I'd completely spaced off the SheetViews collection. This should turn off gridlines for an arbitrary Worksheet object:

Private Sub TurnOffGridLines(target As Worksheet)
    Dim view As WorksheetView
    For Each view In target.Parent.Windows(1).SheetViews
        If view.Sheet.Name = target.Name Then
            view.DisplayGridlines = False
            Exit Sub
        End If
    Next
End Sub

The ActiveWindow is a member of the Windows objects collection. As with any collection, simply refer to the actual window by name rather than by specifying the active window. eg

Windows("My Workbook.xls").DisplayGridlines = False