The anti-virus software I'm using is "AVG Anti-Virus Free Edition". It's free for home and non-commercial use. I've been using it on all of my computers (2 desktops and a laptop) for over a year. I think it's good enough for personal use.
You can download the free version here:
After you download and install the software, make sure you "set your AVG Free Edition to be updated automatically." The following quote is from the FAQ page item #653:
How to set your AVG Free Edition to be updated automaticallyI set the time to "between 8:00 and 9:59" and also check the box for "If Internet connection is not available, check when it goes on-line."
- run your AVG Free Edition by double-clicking the AVG 7.5 icon on your desktop
- click the Service menu and choose the Schedule daily update item
- here you can set the time of your AVG's regular daily update
Also make sure you turn on the "Resident Shield." Basically, it will scan those files you are accessing. (Like when you open a word document, it will scan that document on-the-fly) If it detects a virus, it will prompt you for the risk.
Of course, last but not least, you should scan your computer regularly in case a virus sneaks in.