Check .shiftKey which returns a boolean indicating if the key is pressed. Placing this conditional around your switch will prevent any events from occurring unless the key is pressed in combination with shift.
document.onkeydown = function(evt){
evt = evt || window.event;
if(evt.shiftKey){
switch (evt.keyCode){
case 67:
createNewFile();
break;
case 82:
goToRecords();
break;
case 84:
goToToday();
break;
case 36:
goToMsHome();
break;
case 27:
escToCloseOptions();
break;
case 83:
summary();
break;
case 73:
insertRecord();
break;
}
}
};