Make sure you are getting the latest virus definitions (information about the latest viruses) daily with your configuration. This is important, as new viruses are released daily. You will want to schedule a monthly full system scan when you are configuring it. Most anti-virus software does this automatically for you, but it is good practice to verify this.
AntiVir is free for private, non-commercial use.
AVG Free Edition is free anti-virus software for private, non-commercial, single home computers.
Avast Home edition is free for home usage. It requires email registration within 60 days of installing it, and then once every 14 months.