First if you are not going to use the (usually installed) Norton Antivirus please fully uninstall it first.
Now select one that you want to install (Microsoft Security Essentials is decent and free for all Microsoft users).
Keep it updated and set to scan every day.
Install Malwarebytes (free) then update the program and scan with it daily.
If there ever seems to be any operating problems, have her sign up here, and post questions or problems.Please read this general information on how to keep out of trouble on the internet
This page contains much good general day to day information.
Thank You -