If you're certain that those songs are no longer on your computer, check out the instructions/suggestions here.
Music from iPod to computer.
There's also another method I've seen posted. I've successfully tested this across several different Windows PCs.
Open iTunes and select edit/preferences/advanced/general. Put a check mark in the box marked "copy files to iTunes music folder when adding to library" and also "keep iTunes music folder organized", then click 'ok'.
Connect the iPod whilst holding down the shift/ctrl keys to prevent any auto sync, and if you see the dialogue window asking if you want to sync to this itunes library, click
'no'.
Then go to file/add folder, open 'my computer', select your iPod and click 'ok'.
The music files should transfer to your iTunes.
However it appears that no playlists or any other info such as ratings/last played etc will transfer.
If this is important to you, you may want to use other options.
There's
Yamipod. This is a free program that transfers music and playlists etc from iPod back to the computer.