Right. You can do something like:

On Error Resume Next
AddIns("Solver Add-in").Installed = True
If Err <> 0 Then
   MsgBox "Excel's Solver Add-in is not installed, " & _
   "You must install it from the Excel installation CD " & _
   "for this macro to function properly.", vbCritical, "Error"
    Exit Sub
End If
On Error GoTo 0