Recently I republished a popular post about setting up your AutoCAD with the classic workspace (How Can I Restore the "Classic" AutoCAD Interface with Menus and Toolbars?) but the steps changed with AutoCAD 2015. Autodesk's Dieter Schlaepfer recently published an AutoCAD Knowledgebase Article with the solution.
AutoCAD Classic workspace is not available in AutoCAD 2015
Check out more great tips and solutions http://knowledge.autodesk.com/support/autocad/troubleshooting