Most of the Antivirus keeps system protected. But be aware of it, all the software should be original. User based type.
Some of the top best antiviruses are
1. Microsoft essential
2. Shield delux
3. Trend micro titanium
7. ESET NOD32
These are some of the best antiviruses in the market. I suggest you to use Microsoft Security Essential. Because i am using it. It is really a good one which i have ever need.