I'd like to assure you that your account name is unique and no two Windows Live Hotmail users have the same account name. It's not possible on our system for 2 users to have the same e-mail address. If messages are being sent from your account without your knowledge, it's possible that you were victimized by the latest Malware attack. Check out this Solution article for more info about it and for ways on how you can make your account and computer more secure:
http://windowslivehelp.com/solutions/safety/archive/2009/04/29/recent-reports-of-account-hijacks.aspx
Now, if the e-mail address is somewhat similar but not exactly the same as your e-mail address, you need to report this to our Abuse team. Send an e-mail address to this address: abuse@hotmail.com
BTW, I've moved your post here at the Accounts and Password Forum