In addition to changing the passwords, it may be good for you to start using a password manager.
KeePass is one of the best and it's certainly the best one that's completely free.
Aside from just storing your passwords securely, it can also generate highly secure, random passwords AND you can sync it through
Dropbox so that you can use it from all your devices. Also, it's available for Windows, Linux, Mac, iPad/iPhone, Android and even Blackberry, so you can use it just about anywhere. Not sure about other versions, but the Windows version can work from a USB stick that you can keep on your keychain so that you can use it on someone else's computer (e.g. you're visiting friends and want to check your email from their PC).
I use it on Windows (from the USB), Linux and on my Android tablet.