I think this link will answer your question.
There's no event that is called when the screensaver is running. Typically, most of the applications that do work only while the computer is idle are either screensavers themselves, or they run quietly in the background sleeping constantly and only starting to do any real work once the screensaver starts to run. So to answer your question, you'll need to add something to the startup menu, and have it check, say, every minute or so to see if the screensaver is running. If it is, it should start doing the "real" work. Once the screensaver isn't running anymore, you need to make sure you've put facilities in place to stop the work you were working on.
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