A PC grow box is a case from a computer CPU that is re-fitted with lighting and insulation to give plants a controlled environment in which to grow during cold winter months. PC grow boxes also protect the plants, so animals and rude neighbors can't eat your crops or damage your garden. By using this process, you can have a vegetable garden year-round, or begin growing sprouts to have them ready at the start of the outdoor growing season.
Measure the dimensions of each interior side of your PC with a tape measure and cut your Styrofoam sheeting to those dimensions. Only the four sides need insulation, since the bottom will likely be against a solid surface and the top will hold hot light bulbs.
Apply glue to the corners, edges and--sparsely--to the middle surface area of each Styrofoam sheet, using a glue gun. Press each piece into place firmly, then let the glue dry.
Glue your power strip to the "ceiling" of the PC case so the sockets are facing down toward the plants. Plug the four light bulbs into the power strip.
Guide the power strip's cord through the space where the fan once sat and plug the cord into a wall socket.
Place potted plants along the bottom of the grow box. As the plants approach the light bulbs, they will grow outward instead of upward to avoid withering and burning.