Euuuu it is not Windows versus Unix issue. It is more Jabber (yet another IM system, but "open" and based on a standard, XMPP) versus MSN, AIM, Yahoo Messenger, ... (all based on "proprietary" solutions).

For your information, GoogleTalk is based on the same protocol as Jabber so they can interoperate.

How to set up Jabber?
First I would recommend you to go to http://www.jabber.org/. Here you will find detailed info. You will also find their a list of public servers where you can create a Jabber account. Beware, not all server allows gateways to other IM systems.
The server I'm using allows gateways to all popular IM systems, but it is a French one (http://www.nuxo.net/) so unless you're French too (or speak French), you'll have to do some research.
Once you have your Jabber server, login and passwd, create a new account using the Account entry in the control panel of the N800.

One trick: it is easier to fill in your address book using a PC client (i.e.Psi) and then they will automatically be loaded in the N800 once you connect to your server.