Are you wanting to do full blown conversations or just a fire-and-forget it message? Windows has "Msg" built in but it makes bare-bones look boated if you get me, just sends a message to another user elsewhere and done, that's it. Not something I'd want to chat with, but useful for telling your kid to get her butt to bed without yelling across the house. It's in 7 anyway, haven't checked my 8 desktops to see if they finally removed that yet.
As far as a regular chat program goes, haven't used one myself in years, do know a friend of mine uses one called "LAN Messenger", decentralized, the usual features.. not super light but not heavy either (~25MB in the tray), does do a bit of extras (transfers and such) but it's not in your face either from what I saw. Source is there if you're comfortable ripping some guts out, but personally don't see the point, 25MB is nothing.http://sourceforge.n...jects/lanmsngr/