Log user active session time (excluding screensaver active time)

Hello :-)

As part of new regulations in my industry (education) I am required so log how many hours my users spend in front of an active session (i.e. logged in to windows, and no screensaver active) and report this information weekly.

As I see it I have three options:

I could write an app to do it
I could use a commercial solution (if one exists).
I could pull the info out of the AD security logs (I'm not sure if I can get screensaver active time this way though)

How would you do it?

thanks for looking :-)