How to reset Nintendo DS Lite.

Remove the cover for the battery compartment. At the back of the DS Lite, the cover has a small screw holding it in place. You will need a Phillips size 0 screwdriver to remove it.
Take out the DS Lite's battery pack. There is a tab at the top of the battery pack. Insert your fingernail or the end of the screwdriver carefully into the edge of this tab and pop out the pack.
Keep the battery pack out of the unit for at least 10 seconds. If you keep the battery pack out for too short of a time, it will not cause a reset in the unit.
Put the battery pack back. Just line it up in the compartment and push it back into place.
Replace the cover. Replace with the screw.
Turn on the DS Lite. If the unit has properly reset, a screen will come up that reads "Before you begin, you must adjust system settings." You can now enter all new settings and information on your DS Lite. If you do not get the system settings screen, you did not keep the battery pack out long enough. Repeat the steps and leave your battery pack out for a longer period of time.