Application Sharing - Remote Control

After a very long and protracted playing with MSN Messenger have managed to
get all functions of MSN Messenger working but have ONE PROBLEM.

In Application Sharing - I give full control to the other machine inlcuding
Mouse Control etc - but if they try to do anything from their Machine they
get Message "ALAN busy right now".

What settings need to change to give the other User Full Access to my
Desktop.

Any ideas will be much appreciated

Alan W