network messaging service

Not as elaborate, you could use the net send features if you are running the messaging service or a number of differnet free sevices.
http://www.wegabyte.com/winpopup-messenger/

u have to use 3rd party stuff.
i use "quickmessenger", it's free, good tech support.
u can get this from http://pages.infinit.net/mmoose/
it is installed in my xp machines and have retained winpop in a couple of win98 compus.
there are actually others but i found this not distracting to the users.
ciao.