First, you're gonna need the older version of the software...if your iPod was delivered with the desired older software already installed, you p'bly [o]don't[/i] have a copy. If you updated the iPod to the version you're trying to go back to, you p'bly do have the software, in a folder named "iPod Software Updates" (sorry, don't recall where that's located on Windows).
You'll need two files: one is the actual software (named something like "iPod_18.1.0.1.ipsw") and another is a verification file (named something like "iPod_18.1.0.1.ipsw.signature"). With both of those in the "iPod Software Updates" folder, do this:
1) unplug from the internet
2) fire up iTunes, connect the iPod
3) select Restore from iPod's Summary pane in iTunes window
4) iTunes will offer to use the old software; yes yes
5) let iPod reboot after the Restore, while still connected to computer/iTunes; if it takes longer than 10 minutes you may have to force an iPod restart (or more drastic measures).
Since Restore wipes everything out, make sure you save important stuff from your iPod before downgrading!