Are copies of the sent messages in your Sent Items folder ?
A couple of possibilites..
1. System infected (Scan with AV and anti-malware application)
2. Address book and email address harvested by a phishing attempt, malware
For Hotmail, change your password and secret question at account.live.com
And if applicable see
http://windowslivehelp.com/solutions/safety/default.aspx
http://windowslivehelp.com/solutions/accounts/archive/2008/10/25/what-to-do-if-you-think-your-accounts-been-stolen.aspx
...winston
ms-mvp mail