Force An Addin To Be Installed

Is it possible to force an addin to be installed in Excel 2003 using VBA? I have a spreadsheet that uses the solver.xla addin in a vba macro. It works fine on my machine, where I have manually ticked SOLVER in Tools>Addins and manually added a reference. However, it does not work when I send to someone else. It seems that the addin is not being installed. I have VBA code that opens it:

On Error Resume Next
s = Application.LibraryPath & "solversolver.XLA"
Workbooks.Open filename:=s

but even with this, i get a "Unexpected internal error occurred, or available memory as exhausted" when I try to run the following

On Error Resume Next
ActiveSheet. Unprotect
Application.Run "Solver.xla!SolverReset"
Application.Run "Solver.xla!SolverOk", "$D$46", 2, "0", "$L$13:$L$14"
Application.Run "Solver.xla!SolverOptions", 100, 100, 0.000001, False, False, 2, 1, 1, 0.05, True
Application.Run "Solver.xla!SolverOk", "$D$46", 2, "0", "$L$13:$L$14"
Application.Run "Solver.xla!SolverSolve"
ActiveSheet.Protect DrawingObjects:=False, Contents:=True, Scenarios _
:=True

The one way to make it work is manually going into Tools>Addins and ticking it. This is confusing me as the above, along with the code that opens it, should not even require a reference, but it doesn't work without it