Today I found out that my Hotmail has been used to send out spam emails to all my contacts. I don't use my Hotmail, but I need my MSN messenger so I cannot close my account. 

I have change my password, but I'm worried about how the spammer got my password in the first place. Maybe they can get my new password again? What should I do to prevent this?

I have Microsoft Security Essential antivirus install, and my Windows 7 is legit copy.

Any idea what else could be a problem?

Thanks in advance