What do you mean by system window???
I am guessing that you are referring the program system information that is found under start->programs->accessories->system tools->system information. If this is the case you can go to that shortcut and right click->properties. When the properties box loads there will be a field that states "shortcut key" and it should read ctrl+alt+s, just delete that and you will be good to go!
EDIT: I just read the title again and realized that you were talking about this.