You can't sync your music back from an iPod to iTunes, the transfer of music is designed by default to be one way from iTunes to iPod. However there are a number of third party utilities that you can use to retrieve the files from your iPod, this is just a selection. Have a look at the web pages and documentation for these, they are generally quite straightforward.
iPod Access Mac and Windows Versions
YamiPod Mac and Windows Versions
PodUtil Mac and Windows Versions
iPodCopy Mac and Windows Versions
PodPlayer Windows Only
PodPlus Windows Only
There is also a manual method of accessing the iPod's hard drive on Windows and retrieving your songs posted in this thread:
MacMuse - iPod to iTunes
If you are using the iPod in manual update mode, your iPod is safe from being accidentally erased when you connect to an empty iTunes after reinstalling your operating system. However if your iPod is set to automatically update you will get a message that your iPod is linked to a different library and asking if you want to link to this one and replace all your songs etc, press
*No*. Pressing "Yes" will irretrievably remove all songs from your iPod. Your iPod should appear in the iTunes source list from where you can change the update setting to manual.
You can also use a keyboard command to prevent your iPod auto-syncing with iTunes. While connecting the iPod to the computer on Windows hold down the Shift + Ctrl keys (on a Mac hold down the Option and Command (⌥ and ⌘) keys). This will stop the iPod from auto-syncing with iTunes and the iPod will appear in the source list. Wait until you are sure the iPod has mounted, and that it will not auto sync and then you can let the keys go. This may take between 20 to 30 seconds depending on your computer. Then go to Preferences>iPod and check the "Manually manage songs and playlists" box:
Managing Your Songs Manually
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 again.