raceprouk is right, there is an option which automatically changes your status to away after
x minutes of inactivity - it's right above the option to switch your status to busy during a powerpoint presentation. All you need to do is use the same timeout as for your screensaver so they kick in at the same time.
If you have msgplus it also has an
automatic status changer - if it is enabled I think it overrides the other one. You can change these settings from the menu of your main contact window:
Plus! -> Preferences -> General Options -> Miscellaneous