All your screensavers are in windows\system32 and they have an extension of .scr (for example: ssbezier.scr, you might have others).
Just make a shortcut to that file and put it somewhere where you can click on it, like in the quick launch area. I tried doubleclick from Windows Explorer and that works fine, and I don't see any reason why that shouldn't work.
So no command, no download, just basic Windows.
Is there a command or download to get your screen saver to pop up automatically with the push of a button? I have a Dell Inspiron with Windows Xp Home.