I have used Norton in many of its different forms over the last few decades and am currently using Norton 360 for 2014. I also tried switching to PcCilllin and learned an important lesson. You have to be very careful when switching from one anti-virus program to another. You must remove the old one completely before installing the new one, and some of them require special steps to be run to complete the removal. Failure to do a complete removal can result in conflicts between the old one and the new that can slow your PC down drastically.
Another thing I have learned is that some things I would consider a virus some anti-virus programs do not, such as "adware" and "browser hijackers". These often come with some of the freeware that is available, and one of the ways they pay for the cost of that freeware is by putting advertising on your PC. Since this is not a direct attack on your PC or an attempt to get secure information about you, they are often not considered viruses, but they can drastically slow your PC down. If your PC is slow you may want to get the free version of "Malwarebytes" and have it examine your PC for these.
WARNING no. 1: If you are already using an antivirus program, do not bother with Malwarebytes PRO, It will conflict with some antivirus programs.
WARNING no. 2: Do not set Malwarebytes to run automatically and concurently with another anti-virus program. They will get into each others way.
WARNING no. 3: Make sure you understand what Malwarebytes is telling you before taking the action it recommends on adware and browser hijackers. Do a search on the web first.
And, as you have undoubtedly heard before, be very wary of freeware that you haven't researched for complaints about. This seems to be particularly true of free games.