In your User folder, under Music or My Music, you should see a folder called iTunes. inside this are your music folders and iTunes settings.
Note: This is the normal place for the files. iTunes is capable of reading the music library files from a different location, so if you have customized your installation, look in Edit: Preferences: Advanced: Itunes Media Folder Location. Also, iTunes can be set to not import the music files to the iTunes Library when they are added, so you may have to look for folders of MP3 files elsewhere on your hard drive.
You can transfer your iTunes folder from one machine to another, either by networking the two machines for file sharing, by copying the folder to a USB memory stick, an iPod http://support.apple.com/kb/HT1329 , or by burning the folder to a DVD (or multiple DVDs).
Note: If you are merging two music libraries, you will need to be careful to move the individual artist and album folders manually so you don’t overwrite the previous content.
http://www.macworld.com/article/146958/2010/03/move_itunes_windows_mac.html
http://support.apple.com/kb/ht1329
If you are discontinuing using the old computer, don’t forget to de-authorize it for your iTunes Store purchases before you disable the computer. http://support.apple.com/kb/ht1420
Be careful about attaching an iPod to a newly installed iTunes – if you Sync to the iPod it will erase what you have on the iPod. The Apple transfer utility only transfers purchased songs, not ones you have ripped yourself.
There are software utilities for transferring songs out of an iPod onto machine.
Senuti for Mac http://www.fadingred.com/senuti/
and a variety for PC
http://download.cnet.com/1770-20_4-0.html?query=iPod+transfer&platformSelect=Windows&tag=srch&searchtype=downloads&filterName=platform%3DWindows&filter=platform%3DWindows