Backup itunes library

I bought a new 160 gb ipod and I have synced it to my itunes version 8. But as I new to it, I dont know how to make a backup of my itunes library. I havent got any manuals on it? Thank you

Solution directly from the Apple Support Site:

To copy all the items in your iTunes library (including the iTunes Library file that holds all your playlists, playcounts, ratings, and other data) to your external hard drive, follow these steps:

Quit iTunes.
Locate your external hard drive.
Mac OS X: On the desktop or Finder sidebar.
Windows: In My Computer
Locate your iTunes folder:
By default, the iTunes folder is located in:
Mac OS X: /Users/username/Music
Windows: \Documents and Settings\username\My Documents\My Music\
If the iTunes folder is not in the default location listed above then you can find where the iTunes folder is located by following these instructions:

Open your iTunes Preferences:
Mac: Choose iTunes > Preferences.
Windows: Choose Edit > Preferences.
Click Advanced.
The location of your iTunes folder will be listed in iTunes Music folder location box.
Drag the iTunes folder to your external hard drive. This can take a while if you have a lot of items.
When the transfer is complete, your iTunes Library will have been successfully copied to your external hard drive.

More information here.

If you move your library and use FireFly (or another DAAP provider) to stream them to local installations of iTunes, then no you won't need to be constantly entering your WHS user id and password. Though you shouldn't have to anyway; to use WHS seamlessly you really should sync your ids and passwords between all of your local PCs and WHS.

if it's just files, then transfer to an external hard drive or burn to CDs. Then when you put them back onto your PC, drag and drop into your itunes window.

Not sure about playlists, but i think that they can be exported as i did this when i changed PC earlier in the year.