[Atomic2Mod]

it will keep your sprint account settings, so you won't have to call Sprint after a hard reset.
But it will clean up your phone from anything the previous owner did to it and give you a clean slate. Any third party program the previous owner had installed, any tweaks and costumizations he did, will be gone.

You can re-install any apps you miss after the hard reset, if you have them or know where to find them again on the net.

You can perform a hard reset by going into Start/Programs/Clear Storage
It will give you the option to either format the main storage or the internal storage.
The internal storage is just like a built-in sd card.
The main storage is what you need to clear to get your phone back to factory settings.