Unable to change default screen saver

I am having a problem with a couple of Vista system which are
activating the screen saver after 15 minutes.

I am logged on as an administrator account and when I attempt to
change the settings on the screen saver the selections are all greyed
out and I am unable to edit/change the default. This behaviour is the
same on both Vista systems which are on my household network linked to
a Windows 2003 server.

Group Policies are not the cause.

All updates have been installed.

On one system I have OneCare installed and on the other I did not
install it

This is a real pain as my media centre Vista system keeps turning off
the screen every 15 minutes when watching movies

Any suggestions?