which software is best?
I am not the authority on anti-virus software. I can only recommend those programs that I have actual experience with, and that I personally find useful. My criteria for any antivirus software I use or recommend for my customers are:
1. Small footprint
Protecting your computer from a virus is pointless if the protection you use consumes so much memory and processing power that it slows your machine to a crawl. Not everyone has several gigabytes of memory and 4 processor cores to throw at something as simple as scanning for malware.
2. Automatic updates with ZERO user input
In the real world, if people only need to click once to be fully protected, half of them won't bother. Any virus scanner used in real homes should automatically download virus definitions and update itself at least daily.
3. Minimal false positives
Notice it's not "maximum detection", but "minimum false detections" that I emphasize. Any decent virus scanner can detect and stop any virus for which is has an updated definition and solution instructions (thus #2 above). If everything you do causes an alert from your anti-virus software, you will soon learn to ignore every alert, making even the best software useless.
With the above criteria in mind, my current favorite among anti-virus software packages is Microsoft Security Essentials (also referred to as simply MSE). It is 100% free for the fully-licensed version, it integrates seamlessly into Windows, and it self-updates without any user input. I use it on my own computers, and have used it to clean computers that have already been compromised and infected.
Among 'geek circles', there are also 3 other very popular free anti-virus scanners that receive plenty of praise for the same reasons I prefer MSE. Avira, Avast, and AVG all offer versions of their software that are completely free. They also offer packages with more features, for prices ranging from $20 to $90 per year. All of the 'premium' packages use the free virus scanner, just with more options added for a price.
All four of these recommended free solutions handle updating well, have few false positives, and don't use up much memory or processor power. All of them can be (and frequently are) rendered moot by the end user simply clicking on things that they shouldn't.
There are also a few popular anti-virus products that I would not recommend. Some of them were excellent products years ago, but have become "bloatware" of late, consuming too much computer resources to offer too little protection. I won't post any "black list" of companies or products; suffice to say if it isn't mentioned on this page, I don't recommend using it for the home environment.
There is no 100% effective way to prevent viruses. However, with safe habits, a fully updated operating system, and good anti-virus software, you are far less likely to have problems than the average computer user.