How to program a Arduino Nano(Bricked) with a working arduino nano
I have a page about programming a bootloader using another Arduino.

Basically you grab the code from here and then make up wiring from the good device (which you put the sketch onto) to the device to be programmed, like this:

Arduino Nano        Target Nano

D10 (SS)            Reset
D11 (MOSI)          D11
D12 (MISO)          D12
D13 (SCK)           D13

Gnd                 Gnd
+5V                 +5V