What browsers are supported?

Ajaxbber has been tested on Firefox, Konqueror and Opera. Some minor things seem to work differently between them, but ajaxbber is fully usable.
However, Internet explorer displays divs in his own way, css will have to be modified for him. Actually, it is very ugly :P,but I cant't test it, I don't have any MS Windows at home.

What do I have to install in client side ?

Nothing. You just have to use a compatible browser, with javascript enabled.

What do I have to install in server side ?

You have to install (or use) a jabber server supporting HTTP binding or polling.

Ejabberd, an open source jabber server, is a good example. To configure it, it is the same way as jwchat.

Does Ajaxbber support page reloading?

Not yet, but it is a planned feature. The aim is to be able to add easily Ajaxbber in any website (like gmail does with embedded gtalk)

I changed mootools/jsjac/windoo library with another version and nothing work anymore, why?

Actually, jsjac and mootools does not work together without modification. The cause is the famous problem with "for var i in obj", and jsjsac redefined some document getters.
The easiest faster way to understand is to diff theses files with original ones.