You'll need adb setup on your computer, and you'll need to cd your way into the folder with adb.exe. Forgive me for hotlinking, but this will make it easier: Download this file
, courtesy of NexusSHacks.com (I believe it is Zedomax around here). I would advise to unzip the contents in this file to your C:\Nexus directory, so that adb.exe is at C:\Nexus\adb.exe.
Now, plug in your phone. Open your start menu and search for "cmd" and open up Command Prompt. This is the basic Windows CLI (Command Line Interface).
First, you'll need to open the directory of the Nexus folder you created earlier. To do this, type in the command prompt:
Now, you want to make sure adb sees your phone:
The adb service should start and if it sees your phone it will pop up a serial number. If there is no serial number, you can't use adb while inside android and you'll need to tick the option or find an alternative way. Assuming it finds your device, you are good to go.
In order to copy the /sdcard folder of your Nexus S, you will use the pull command. We will throw all of the files into C:\Nexus\sdcard, for reference. So in the command prompt, type:
adb pull /sdcard .\sdcard
It looks a little confusing but don't stress. Just make sure you type it in exactly (dot, correct slash, etc). This will take a little while since it might transfer up to ~15GB worth. You'll see a new line with "C:\Nexus>" with a flashing cursor when it is done. Then you can view all the contents of your sdcard at C:\Nexus\sdcard and do with them as you please.
Also note, for this you'll need up to ~15GB of free space on your C:\ partition, make sure you have this before you start. Good luck