the program to use is yamipod:
http://www.yamipod.com
first, install iTunes latest version on the new computer. once that is sorted, you'll see that there is a folder on the hard drive in C:\documents and settings\[username]\my documents\my music\itunes\itunes music. remember this folder.
now, download yamipod. connect your iPod, and iTunes may pop up and ask if you want to link the new iTunes library up. say NO. then close iTunes.
run yamipod. it'll load all the songs from your iPod. goto file -> copy, and copy all the songs, to the folder i gave above. you can also export your playlists too, if you like.
now, close yamipod, and then open iTunes. click file -> add folder to library. choose the above folder. it'll take a while to add all your stuff. once all is done, click advanced -> consolidate library.
done! now you can right-click your iPod in iTunes, and update it.