The simplest way is to open Windows Explorer (the Windows equivalent of the Finder,) go to the Tools menu, and click Folder Options, go to the 'View' tab, then click 'Show hidden files and folders', and DE-select 'Hide protected operating system files (Recommended)'.
Then plug in the iPod, and wait for it to appear as a drive letter (if iTunes launches, make sure to tell it not to erase the files, and then turn on Disk mode.) Open the drive (I'll assume it shows up as 'E:',) then open up the 'iPod_Control' folder (it will be grayed out because it's hidden.) In there is folder called 'Music'.
This 'Music' folder contains all of the songs, but it stores them in a bunch of 'Fxx' folders, with really goofy names. Drag each 'Fxx' folder onto the iTunes window. If you tell iTunes to keep your iTunes Folder organized, then it will re-name them to the proper names as it imports them.
You'll have to re-authorize the computer, and you'll lose all your playlists, and metadata (play count, last played, etc.) but the actual music files will be there.
There are also pieces of commercial software that can 'recover' your library from your iPod, but I don't know the names of any of them off the top of my head (since I wouldn't pay money for them...)