detect if a new process has started

[sheduler]

How to detect if a new process or application has been started system wide? One way is to get then full list of processes and the just wait for changes. But it requires a timer, what I want to bypass... Maybe there are some other ways?
And the same with the active window. One way is to set Ontimer GetForeGroundWindow and just wait for it changes but again it takes much more cpu... Maybe here is an other answer too?