Hi.
Havnt got my pod with me today, but off the top of my head:
In itunes, under -EDIT, then -PREFERENCES, then the tab titled-IPOD (You must have your ipod hooked up to enable this tab).
There should be various selectable options on the left hand side.
One is Automatic updates (default) another is manual update.
Select MANUAL. This will stop itunes from adding or deleting music from your ipod. You can go ahead and delete music from your computer without any probs.
However... If you ever select Automatic again and have your ipod hooked up, it will delete your music.
On manual update: To add music from itunes to your ipod, you will need to drag the music from your itunes library to the ipod library.
Hope this helps.