Hi girls! Actually, you have to install the Windows Live Messenger AND the other links that Amanda posted - and after you install those you will have the option to have tabbed chat windows for each person you're chatting with. I don't *think* you can send screenshots when you're in a group chat - we tried and it didn't work, but it may have been us doing something wrong.
It has some really cool features - like if you choose to send your active window, it has a countdown (you can set the time) and it gives you a few seconds to choose which window you want as your active window. You can also select what you want to send (as in draw a box around something with your mouse). I've been messing around with it all afternoon and don't have quite everything figured out but if you post questions here I'll check and see if I've figured it out yet LOL