Right, ITunes shouldn't let you do this, its called 2 way synchronisation and its not part of iPod features. If you want to do this, use Windows Explorer with the ipod plugged in and look for an entry marked removable disk.

Double click this, it'll be the ipod. Make sure you have the option to view all files and folders turned on. There should be a music folder thats normally hidden. In here are lots of numbered folders. Drag the lot across onto a folder on the PC. You have all the files now on your machine. Bit of a bollock ache, but the easiest method to get the job done.

Cheers

Matt