|Is your computer a brand name like a Dell, compaq, IBM, etc? If so, what's the model? If it's something generic look on the motherboard for a model number. Maybe we can get some info on it.|
"The box in the back" is probably the power supply. There's going to be one large connector with 20 wires. That provides power to the motherboard. There's only one place it fits and only one way it fits. There's going to be several 4-wire connectors. Those go to the drives--hard drive and cdrom. They only fit on one way also. If you have a floppy drive it too takes a 4-wire connection but it's going to be smaller. There will probably be more 4-wire connections than drives so some will be unused.
Post back info on the system or motherboard and we can see what other connections it may need.