Source: http://lifehacker.com/5962823/hack-a-dead-pc-power-supply-into-a-rechargeable-lantern
Computer power supplies can burn out after several years and unless you're an advanced electronics hobbyist they're not worth trying to rebuild. Instructables user 300mpg notes that You can reuse many of the components as long as you're careful to follow proper capacitor safety procedures. To make the lantern you'll open up the power supply and remove all internals except for the power switch, power plug, and wiring. Cut out the fan grating and fit a 12v landscape lamp, but don't permanently attach it until you test the wiring. Wire the switches, lamps, and battery together and test the function of the lamp. Once that's done cut two openings in the top of the power supply case for a handle made from the the wiring bundle that connects the power supply unit to a motherboard—zip tie the ends together from inside the power supply unit and you'll have a comfortable handle for your lantern. Now you'll just need to cleanup the case and glue the battery and lamp to the casing; adding foam spacers between the case and the battery would be a good idea and used mouse pads work well for this..
From the original design you would connect a 12v sealed lead acid battery charger to the positive and negative connections on the power supply plug. As commenters on the Instructable pointed out, mounting a battery charger inside the case might be a superior design as then you already have the plug and can reuse a standard 3-prong computer power cord.
This seems like a fun project that makes dead components useful again and is worth doing as long as you follow all electrical safety procedures.
Hack-A-Lantern: Recycled Computer Power Supply Flashlight | Instructables