If you have created a new Windows user account you need to log out and then log back in with your original user name. By default each Windows profile will have an independent iTunes library.
If instead you managed to create a new library within the same profile. click the icon to start iTunes and immediately press and hold down the shift key, keep holding until asked to create or choose a library. Click Choose and navigte back to your original which should be at <User's Music>\iTunes.
Your question as posed was:
What is the best way to manage multiple iPods using the same iTunes {library}?
in which case Selected Playlists is probably the best approach. Simply create a playlist for each device, Alice's iPod, Bob's iPhone etc. and sync the device with the appropriate list.
tt2