Thanks, Nuru!
If you are an owner or a member of a yahoo group, then you already have an ID.
One place for info is this:
http://messenger.yahoo.com/?i=1
and click on "New to Instant Messenger". That tells you all about the installed version.
I would use the Web Messenger instead of the installed version if I did not have a good firewall at home (I do, so I use the regular one.)
The web one is more limiting, but you can use it especially for a planned time & meeting, it should work. Go here
http://messenger.yahoo.com/?i=1 , scroll down to the bottom of the page and choose "Launch Web Messenger".
You'll have to add our screen names to your buddy list. Mine is in my profile, as is immy's; I don't know about lmp.