Norton/McAfee/AVG/Kaspersky all these paid softwares I tried for past 7 years.
They will mainly eat system resources & gives more false positives.
Finally I sticked with Avira.
The only annoying thing with avira is daily pop-up, comes once in a day.
For me it is ok. 1 pop up is better than 50-60 pop ups about threats/ infections.
Never Install more than One antivirus Software at time in the system.
One more choice is Avast.http://www.avast.com/free-antivirus-download