XP remote desktop doesn't kick the person out, it just starts a new session, seperate, so what you're actually asking about is controlling a remote user session already logged in... right?
I haven't heard of any problems with VNC, I used it to administer my mother's PC. Read VNC website they probably address the security concern...
no your right it doesn't kick them out, but it does lock the computer. if they unlock it, it locks the remote session.
I found a way to use netmeeting so that administrators can connect without having to get permission. you have to activate remote desktop sharing