Long ago I used Norton Antivirus to keep my computer protected agaist viruses. It was good at what it was supposed to do, but each new version seemed to demand more CPU (or other resources) that my day-to-day tasks were impacted negatively with slow response time. What finally made me switch was the annoying reminder windows to renew my software. Long story short, I found out about AVG antivirus and have never looked back. There is even a FREE version here.