I installed Miranda IM and found it is powerful to support multiple IM networks. Light and easy to use.

Miranda IM is a multi protocol instant messenger client for Windows. Miranda IM uses very little memory and is extremely fast. It requires no installation and can be fitted on a single floppy disc. Its powerful plugin system makes Miranda IM very flexible. Only the most basic features are built in, but there are currently more than 350 free plugins available for download that allows users to extend the functionality of Miranda IM. To see how users have customized their client, check out the screenshot section.

One issue I found is that it cannot add multiple accounts for given network.  The solution I found now is  to copy the network plugin dll and save as another file. In this way, I can add multiple MSN accounts into it.