Well, I think we need to do some checking before I make any recommendations. Lets check that your machine has indeed not been taken, and is not being used as an SMTP relay - (email server)
I'll assume Windows as your OS, so open a command window, and at the prompt type the following command 'netstat -a'
Look under the Local Address Column and look for ":25"
If that exists, then your machine is likely acting as an SMTP relay, and we will have bigger problems to fix it. I have seen this few times on Windows, and you may well notice your network speed has seemed very slow lately.
If not, then it is likely that your email address is being used as suggested previously. I would however like to see one of these emails, with the full detail log turned on so I can look at the SMTP headers.
On the Topic of passwords can I recommend Lastpass to generate complex passwords which are non-dictionary originated. You have one global password which controls all your other web accounts if you use it wisely. Make your global password a complex phrase as it will hold the keys to your digital life. I use two stage authentication with Lastpass, but that is because I'm a paranoid penguin ;-)