Things to consider when selecting an Instant Messanger Program.
Who are you looking to communicate with? Do they already have an IM program? You would need the same one (or an account on the same network) to talk to them.
Options include:
Windows Live Messanger http://get.live.com/messenger/overview
AOL Messanger http://www.aim.com/
Yahoo Messanger http://messenger.yahoo.com/
ICQ http://www.icq.com/
They have different feature sets, for example Windows Live and Yahoo have webcam capability and I do not believe that AOL or ICQ do. If you are needing certain features, that could affect my recommendation.
There are also programs that combine many networks into one program and allow you to talk to inidividuals who use any of the supported networks. One such program is Trillian
http://www.ceruleanstudios.com/
Overall, Id recommend Windows Live. http://get.live.com/messenger/overview
I get spam with Yahoo
ICQ use to be very popular but is no longer
AIM does not have very good features and has very annoying advertising sometimes.