For such as these, simply record a macro. That gives me
 
Sub Macro2()
' Macro2 Macro
' Macro recorded 19/05/2010
AddIns("Solver Add-in").Installed = True
    AddIns("Solver Add-in").Installed = False
End Sub

Add the relevant bit to an Open/Close event

[Code]
Private Sub Workbook_Open()
AddIns("Solver Add-in").Installed = True
End Sub

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