|There are other free AV programs, such as "Microsoft Security Essentials" and "Avast". Both are quite adequate unless you are an unsafe browser.|
It's worth having MalwareBytes on board (to scan when you suspect an issue). This can often find what AV's miss.
Another one, which uses no resources and does not run in the background, is SpywareBlaster. This puts kill bits in the registry to protect you from known nasties. It only runs briefly when you update it about once a forthnight.
Don't have more than one program running in real-time as they could conflict. Best let just your AV do this.
Ms Hulot's boat