Before you can install another antivirus program, I recommend you do a google for 'Norton Remove'.
This will completly remove Norton and allow your system to load another antivirus program.
I have never used this prgram,but there are plenty of alternatives out there to chhose from.
My personal choice is Stopsign, for which I pay about £25 per year, but to date, it has always done the job, and when I run (as a check) a program like a2free, it rarely finds any problems.
But as the man say's 'You pays your money and makes your choice'