For Computer gurus

[NYFAN]

Ok, here's my question

I recently started getting a number of MailerDeamon messages on my aol account, and just ignored them... then however I looked in my sent messages folder and noticed that someone had apparently been sending spam messages out using my account. Now, my question is, will a simple password change fix this (if so, should I change all of my other major passwords as well??), or is someone e-mailing via my account through a means other than logging in. Reason I ask, I really can't figure out how someone would have gotten my password over the last few days. It makes little sense in terms of timing. I haven't noticed any other abnormalties in my account, or accounts with similar logins/passwords. So, basically I'm just looking for some advice. Thanks.