how to detect if User is active in application? mousemove, keypress...

I basicly need to know how to detect if a user is infact using the application, if not i will lock it after a certain period of time. (i have the time part going which is easy) I also made this work my putting code for each mouse_move event for each control and form but some controls dont have this event and also the user might just be usig the keyboard...

What i need to do is exactly how windows handles its screen saver and starts it after a specified time lapse of inactivity.

Thanks,
Dan