If they are sending from your account, change your account password at http://account.live.com
Contact Windows Live Hotmail Support - https://support.live.com/eform.aspx?productKey=wlmail&ct=eformts
If they are spoofing your address, meaning your address appears as the From line in Spam sent to you and others, it isn't sent from your account, though bounced messages will be returned to you. If this is the case, ignore the bounces as there isn't anything you can do short of getting a new email address. The spoofing should stop after a while and they use another address.
-steve