easier way to find your serial: open iTunes. Then click the menu tab "iTunes". Then hold down the control key while you're clicking onto "About iTunes". Now you you see a slightly modified About iTunes page, with contains the serial number of your last connected devices. (last iPod and last iPhone). If you see the numbers, press the spacebar, so the screen doesn't move anymore and you can write your serial down.
One problem: if you sister also connects her iPod with the same computer then it might show hers (depending on which was the last one!)
EDIT: her's another link you might find useful:
http://support.apple.com/kb/HT1459