Spammer Accessed Email Account

Hello,

Somehow a spammer got access to my email. Any ideas how? Here's some stuff I use:

Windows 7 x64
Open DNS w/ all extra security options checked (phishing protection, basic malware/botnet protection, block internal ip addresses)
Wireless Router w/ a firewall MAC address filtering and WPA2 password protection
Kaspersky Internet Security 2011 (currently doing a full scan was on automated scanning daily)

I haven't opened any strange looking email attachments or gone to any unknown websites. I'd like to think I'm rather computer savvy but I'm stumped on this one. Thanks!

Edit: forgot to mention the spammer gained access to a Yahoo! Mail account.

Edit 2: the browser I'm using is Google Chrome.