Actually, jwz wrote a screensaver engine into which actual screensavers can be plugged. He wrote the part that detects idle time, locks the screen, loads the screensavers, etc.

You'll also note that he has a disclaimer on his site:

There is no Windows version of xscreensaver, and there never will be. Please stop asking. That goes double for the ``xmatrix'' mode. No. The answer is no. I don't use any Microsoft products and neither should you.