Defrag your computer. (How To: http://windows.about.com/od/maintainandfix/ss/defragment.htm )
Also, you might want to check your connection from your keyboard to your computer. If you use a wireless keyboard, update the batteries. It also might be time to get a new keyboard if your current one is old - keyboards do wear out fairly quickly.
Limit what you're running in the background while in a live chat, that might have something to do with it.
Lastly, check your internet connection, that can also cause a lag.