Generally most users stick with MSN Messenger, AIM (AOL Instant Messenger), Yahoo! Messenger, and ICQ.
I think the best way for us to help you decide on an IM client is for you to tell use what features you require (voice, webcam, logging, whiteboard, etc) and what its main purpose is (business, personal, etc).
Also worth reviewing is our
All about IM section. This will tell you the basics about the 4 big IM clients. Browse around the site, perhaps it will help you get a grasp on whats out there and what's popular. Also remember that most IM clients require that both parties be signed up with an account with them in order to communicate. This is where alternative
interoperable clients such as
Trillian come in.
Also, to answer your MSN/Windows Messenger question. Yes, they are two separate clients. MSN Messenger is geared toward personal use, as its full of features such as games, display pics, and animated emoticons. Whereas Windows Messenger is only compatible with Windows 2000 or XP and is now developed more for corporate use, with features such as whiteboard sharing and remote assistance.