Setting up permissions with multi user accounts

I am running windows xp pro from a home computer with multi users. I would
like to be able to set up permissions/user groups to allow adults to be able
to use programs which the children are not able to see or access. Can anyone
help me to do this is laymans terms? I have read several places which has
lots of info on how to do this - but I walk away scratching my head! Also,
how can I install a program such as an antivirus program for run for all
users in the "backround"? Norton wants everyone to scan thier files, but it
is set up to run for all users accounts anyway, so there is no need for it to
run under each user account. Thanks!