I installed a wireless system at home like the one below.
http://www.smarthome.com/73902.html
You can purchase as many motion/window/door sensors as you want. The unit is 29 zone capable. You can sign with a local alarm monitoring service so it's monitored 7/24.
Of course you can have ADT / Brinks intall one for "free" but you need to sign up for their service 2~3 years depending on their terms.