It is not necessarily the case that the spam was sent from your hotmail account. It is possible that once your contact list was harvested, the messages were then sent from some other mail system, and your email address was "spoofed".
To know for sure, you need to get the full email "header" from one of your friends who received the spam. This will show us the various machines and actual email systems that sent the spam.
Here's a tutorial that explains how to get the headers from various email programs.