Thank you too guys!!!! robdog2004 and frauhottelmann, you really know what you're talking about. Here's what I did: following your advices I deleted (first I always backup of course) the ml_ipodxxx.ini files. Then when I reconnected the nano I got the initial message from ml_ipod asking me what type of iPod I have. Then I selected the correct one.... and again I couldn´t see my songs. Ok, no problem, but I had set up the right device. Then some few songs displayed there when I started iTunes I think, I'm not so sure, but the thing is I still couldn't see most of my almost 400 songs. So I did a backup: I copied every single file even the hidden ones to a backup folder in my hard drive. Then via Windows Search tool I gathered all mp3 files and deleted them. Then when I started Winamp again, i still could see the few songs as if they were still already there. I remembered when you leave an item on the list even if it's not physically in the iPod anymore, and you try to reload it, you mess it and it's not playable anymore until you delete the file from Winamp, disconnect and reconnect the iPod and reload it. So to avoid that long step I simply "removed" those files (only removed them from the list, as they were not actually in the iPod anymore).
Then I simply reloaded the files from the backup again and... voilà!!!! Everything is just as when I started. Except from a few files here and there which are untagged, but it's OK, I can manually tag them again. That's a minimum issue compared to what it could have been.
Just an advise for the developers (yes, it's easy to ask, but not that easy to do I know) it'd be great if you could include some option on next ml_ipod versions that give you the choice to re-select your iPod model, and that option would only need to (for safety backup first ml_xxxx.ini files) delete those two ml_xxx and the pmp_xxx files and ask you to reconnect the iPod. That simple pre-programmed step would avoid these headaches, don't you think?
But again, thank you soooo much for your help, I learned new things now (it's great to learn from errors) and of course, my most sincere congratulations for your developing work. I'm just starting programming but maybe in the future I can collaborate with your projects.
As soon as I get my paypal account I'll start donating for this project. I hate iTunes. You're the best!!!