this may help (to check whether solver.xla has been opened):
this talks about explicitly using solver.xla as a reference in XL2000, so I suppose that whether or not it is present may serve as a useful check.
here's another MS help doc that might be useful:
' Macro to call Solver main dialog box.
Sub MainSolverDialog()
' Turn error checking off.
On Error Resume Next
' Test if Solver add-in is already open.
If Application.IsNA(Workbooks("Solver.xla"

.Name) Then
' If add-in is not open, then open Solver.xla.
' NOTE: You may need to adjust the path to Solver.xla.
' For example, the path may be
' C:\MSOffice\Excel\Library\Solver\Solver.xla.
Workbooks.Open ("C:\Excel\Library\Solver\Solver.xla"

' Run Solver auto open macro to load add-in.
Application.Run ("Solver.xla!Aut

pen"

' Ends the If statement.
End If
' Turn error checking back on.
On Error GoTo 0
' Run the Solver macro to display the main Solver dialog box.
Application.Run ("Solver.xla!Do_Main"
' End the macro.
End Sub