To use desktop applications as your defaults, open the Charms bar and head to Settings > Change PC Settings > Search and apps > Defaults to select your desired default applications. You'll need to download some killer desktop software first, of course.
If you like using the new Store apps on your tablet, but prefer using traditional desktop software when using your tablet as a PC, try Stardock's $5 ModernMix. It lets you use confine those touch-first apps to traditional desktop windows when you're using your tablet as a PC, at the mere touch of the F10 button on your keyboard or menu bar introduced by the software.
Beyond MetroMix, you can also use Windows 8's built-in Snap multitasking feature to run Modern-style apps alongside the desktop, or just display Modern apps on your tiny tablet display and the desktop on your larger display. Both screens can be used at the same time in a tablet-as-PC setup, which can create some handy scenarios if, say, you want to use the touchscreen potential of the tablet itself to create diagrams, take notes, or mark up PDF files.
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