So if one has fallen in love with the screensaver, but don't want the hazzle of turning it off before starting a defrag, then this can be a help.
Update the following STRING values :
Note the screensaver will not be disabled if the Defrag is run as a scheduled task.
[HKEY_CURRENT_USER \Software \Microsoft \Windows \CurrentVersion \Applets \Defrag \Settings]
(Default) = "YES"
DisableScreenSaver = ""
Also Read Defrag partitions to speed up file access
More Info MS KB229057