Karen has been running her PC without an AVS and could install McAffee or Norton. Leo says he hates both of them, passionately! Since Karen doesn't have much money for this, Microsoft offers an excellent free option called Microsoft Security Essentials which will do the job quite nicely. She should install that, keep Windows up to date along with her browser and other software. Then all she really has to worry about is her activity online. She should also be careful not to fall for the tricks, even if they seem to be from a friend.
Leo recommends Google Chrome or Firefox as a browser. Also, Chrome has the benefit of automatically updating Flash because it's built in.