You want that in your user's account.
How do you start up your gui? If you use startx, it's simple. Make a file in your home directory, .xinitrc
and put something like this in it
# fancypiper box user phil .xinitrc file
exec xscreensaver &
#exec gkrellm -w &
You can do that with the file your login looks at if you use a gui login
Read for the login manager you use.