Local mail folders on your PC? Probably a virus or trojan. They've got complete access to read any file on your system.
Webmail? Could be a password cracker logging into your account.
Social engineering is a big one these days. Facebook has a function that allows it to contact all your address book contacts and invite them to be friends. All you have to give it is your email login and password credentials. Of course, it then has access to your email and contact address book. As little as i'd trust Facebook to be careful with this information, i've seen hundreds of rogue apps that offer the same functionality and have zero or less trustworthiness.
I got one of your spam emails. Delete all cookies from your browser. Do a complete virus scan. Then log in and change your passwords. I had to do this not too long ago. Same thing with my brother in law. Yeah it sucks.
I would recommend using a spyware cleaner such as malwarebytes.org AND superantispyware.com. Keep using them until everything is cleaned from your system (start with Malwarebytes and once it shows nothing, move on to Superantispyware.
Appears to be my Hotmail account, which, it appears, shares the address book with Windows 7 Live Mail, which I hate!
This is something I have posted a lot of times:
The number one security measure one can take on a PC is to avoid Internet Explorer, Outlook, and anything Microsoft refuses to separate from it's OS. Microsoft knows that the biggest vulnerability of it's OS has always been it's selfish insistence of tying it's "services" and Browser to it's OS, but they will absolutely never change it. Instead they would rather issue 6-10 security "fixes" a month.
I would switch to Yahoo if you must use an online e-mail and stop using Internet Explorer, Outlook, Hotmail, etc. Use Firefox and Thunderbird instead.