is ur client is an applet. or application..
one solution is to use a dll file using JNI the most perfact way ( like MSN )
but if u dont want to use JNI then go for just
intentiating new instance of the private applet class it will automatically comes on front of all other applications.
one more way is to just send that private message to that perticuler client not other on the same applet window rather intentiating new one.
let me know any help u require about chat..!