This iPod is linked to another iTunes music library????????

Your iPod is set to update automatically, in this mode it can only be linked to one library. If you transferred your previous iTunes library to the iPod then presumably both have the same content (songs, playlists etc). If that's the case then choose Erase and Sync and link your iPod to the new library. If iTunes is different from the iPod then when you connect and get the message that it is linked to a different library and asking if you want to link to this one and replace all your songs etc, press "Cancel". Your iPod should appear in the iTunes source list from where you can change the update setting to manual and use your iPod without the risk of accidentally erasing it. Check the "manually manage music and videos" box in Summary then press the Apply button: Managing content manually on iPod

With the iPod in manual mode you can use it with multiple libraries, you can drag and drop songs from the new iTunes to the iPod icon in the source list: iPod 101: Fill 'er Up

Once you are safely connected there are a few things you can do to restore your iTunes from the iPod. If you have any iTMS purchases the transfer of purchased content from the iPod to authorised computers was introduced with iTunes 7. A paragraph on it has been added to this article: Transfer iTunes Store purchases using iPod

The transfer of non iTMS content such as songs imported from CD is designed by default to be one way from iTunes to iPod. However there is a manual method of accessing the iPod's hard drive and copying songs back to iTunes on Windows. The procedure is a bit involved also it was written long before iTunes 7 so it requires a little adaptation as the iPod preferences are no longer in the location described. Also it won't transfer playlists or videos as far as I know. However if you're interested it's posted in this thread: MacMuse - iPod to iTunes

If you prefer something more automated then there are a number of third party utilities that you can use to retrieve the music files and playlists from your iPod. You'll find that they have varying degrees of functionality and some will transfer movies, videos, photos, podcasts and games as well. Have a look at the web pages and documentation, this is just a small selection of what's available, they are generally quite straightforward. You can also read reviews of some of them here: Wired News - Rescue Your Stranded Tunes

TuneJack Windows Only
iPod2PC Windows Only
iGadget Windows Only
iDump Windows Only
YamiPod Mac and Windows Versions
Music Rescue Mac & Windows
iPodCopy Mac and Windows Versions

Whichever of these retrieval methods you choose, keep your iPod in manual mode until you have reloaded your iTunes and you are happy with your playlists etc then it will be safe to return it auto-sync.