I use Miranda IM, it's a lightweight (no bloat) multiple protocol messenger.
It's Plugin based and the user community there (tech support) is excellent. It's a bit overwhelming to configure and stuff at the beginning but it's worth it.
From the homepage:
Miranda IM is a multi protocol instant messenger client for Windows. Miranda IM is designed to be resource efficient and easy to use. It uses very little memory and requires no installation. Just unzip and run!
http://www.miranda-im.org/about/
Have a look!