Check to see if the screensaver is on your system
C:\Windows\System32\Mystify.scr If you find it then double click the file and that should open the screensaver. Click once to close it!
When you say they aren't previewing, what happens if you set the time open the screensaver to 1 minute then see if it works!
But what about the other windows 7 screensavers, do they preview and run ok ?
Bubbles, Ribbons etc.