Antivirus is important but without update it serves no purpose. Make sure you enable automatically update your virus pattern on day to day basis. There are basic and free antivirus out there, if you have an outdated or expired antivirus, you can download free edition for home users by Grisoft AVG, it has the most basic feature – Antivirus. No Rootkit scan other tools. But I personally think that is sufficient and not bloated. Bloated antivirus can slow down your system. You can download this Antivirus at http://free.grisoft.com
Another good and Antivirus is from Kaperskly lab, you can obtain free Kapersky license here . Not to worry just input your email address and wait for the key to be emailed to you. The key is valid for six months. After that you can make your decision whether to purchase commercial license. Remember a good healthy system is crucial to protect your important data.