Detecting System idle time

Hi,

Is there an API that can be used to detect system inactivity with respect to user interactions ?

I know this is probably a very simple question, but as indicated, I am a beginner.

I need to make a non-GUI application that will act kind of like a screen saver, but without being a "screen saver". I need to detect
if the system or computer has been inactive for a period of time at which point it will process some other commands.

I know this has to be possible, I just do not know how to do it. If someone has an example of this, if that is not asking too much,
that would be much appreciated as well.

As a sub-question. Would Windwos screen saver routine cause problmes with this. By this I mean, say I want to detect
system idle of 20 minutes and then do my processing. But Windows is set to lock the screen in 15 minutes. Will my
background process ever occur.?

Thank you,

Eliot