You could both just sign up for an IM account at one of the many providers
And you have many choices of IM client. If you both want to use the same client there are many that have builds for both OS's. Pidgen is easy to use and setup. BTW, this wouldn't be limited to your LAN. Just saying that because you specified it. If you are using one of the public IM services your messages will go out of your network and then back in. There isn't much of an issue with that and nothing you'll need to do to your network gear to make it work, just thought you should have all the info.
Last edited by mglenney; 11-10-2009 at 05:17 PM.