Never ever have multiple antivirus programs INSTALLED on your computer. One will always flag the other. If any action is automated the running program will delete or isolate essential files of the other. Over time (hour, day, month) they will start causing instabilities.
Norton has become obsolete and you will notice a MASSIVE performance gain just removing it and going with one of the other two that have been proposed. AVG doesn't tax your system, but isn't quite as effective as other options.
Microsoft Security Essentials is free and has proven to be superior to even pay services for all but targeted attacks. The reason is because well over two thirds of issues stem from Microsoft's own operating system. MSE tends to more fix 'loopholes' then patch problems.