Hi Friend, i have the experience on this situation, and i want to advise you two way to protect your house but leave windows open: one, to install a home alarm control panel which can alarm in spot and auto dial your phone number, and install a pair of IR baluster movement detectors at the two sides of your window, you can install it outdoor or indoor, one side is receiver and the other side is transmitter, it's wireless between the two sides, if any intruder pass the IR baluster detector, the detector will send a wireless signal to control panel,then the system alarms... TWO, you can install a LED illuminated warning sign(green/red color), which says: WARNING, SECURED BY 24HR MONITORED ALARM SYSTEM!. And you'd better to install the two systems at the same time.
Any question, reply me or email me?

hope you can find a solution