The two major open-source instant-messaging clients out there for Windows are Pidgin and MirandaIM.
I have used both. Pidgin is by far simpler. You simply install it and add your Windows Live account information. MirandaIM is more customizable, layout-wise, and if you're only using it for one Windows Live account then it works very well.
But both of them can support more than one account, they both support tabbed chatting, they're both great programs.
Good luck!