Downgrading iPod Firmware

I want to downgrade my iPod's firmware, nothing I've tried works, I've googled it and everything thing, all the results on google don't work, the apple support site sucks.

I've tried using the iPod updater restore thing in the utilities folder (the one on the CD that came with my iPod) but it says "The attached iPod model is not supported by this application."

What ipod is it? What firmware are you currently running, and what firmware would you like to running???

I am using a first gen iPod nano, I am not really sure which is the firmware and so on, it's the latest one, I know that for sure. I dunno if this helps but it's software version is 1.3.1 and I wanted the earliest Firmware.