>Having a multiselect listbox in a form, how can i activate a context menu with the rightclick of the mouse, without desselecting the previously selected items ???. Normally the right click works as the left click so it deselects the items and select only one.

Have you looked at MouseDown Event? I don't know whether it will fire before or after the Click events but you can intercept the button pressed and have a case statement depending on which one was pressed.