Mine is only "greyed out" if DisplayProperties>Screensaver is set to "None".
Can you choose a different screensaver, adjust the timer, and then choose
your screensaver and click "Apply"?
Start>Run>(type inRegedit> Expand(click the "+") My Computer>HKEY_Current
User>Control Panel, click the word "Desktop". Look/Scroll the right-hand
Pane to find the "string" (entry): ScreenSaveTimeOut.(Pass the corsor over it
to get the full text word.) The value to it's right is the timeout in seconds
(i.e., 600 = 10 minutes).
[The SSTO string may be missing.]