From BlenderWiki

Setting the default scene

You don’t like Blender’s default window set-up? You want specific render settings for each project you start? You want to save your theme? No problem! You can use any scene file as a default when Blender starts up. Make the scene you are currently working on the default by pressing CtrlU. The scene will then be copied into a file called .B.blend in your home directory.

You can clear the working project and revert to the default scene anytime through the menu entry File » New or by pressing CtrlX. But remember to save your changes to the previous scene first, if needed!