alright, what you have to do is go into settings, device, and find a little button that says "forget this device" what that will do is make it so when you plug your zune in, it will connect as a guest. which means that your computer will not automatically delete or add music to your zune. and just make sure you always connect it as a guest if you are ever prompted by zune. just be careful though, only delete the songs off of your computer that you can live without. just incase your zune has a major malfunction and you lose everything on it and you will probably want to cry...
ohh, and when you connect as a guest, you can just go through and add/remove anything you want from the zune or the computer and it won't automatically change anything.
hope this works for you =]