As previous stated, Microsoft Security Essentials is a freebie from Microsoft and is more than adequate for the average user to protect against virus's, spyware, malware. It will auto update itself and also run occasional scans of your system in the background unless you don't turn your pc on for several days. It will then give you a prompt to scan your system. After the initial install, go into setup and make a few setups such as the time/frequency for scans, etc.
http://www.microsoft.com/en-us/download/details.aspx?id=5201