Your Windows computer may have come with a free-trial of an anti-virus program. You may have seen a great deal on an anti-virus suite. You are really better off deleting those trials and not spending a nickel on these products. Modern operating systems like Windows 8 and Mac OS X Mountain Lion have all the basic protections that you need.
Certainly there was a time when these products mattered. Viruses ran rampant on all platforms and operating systems were upgraded at a glacial pace. In 2013, however, Microsoft and Apple push out updates on a regular basis - and both have worked very hard on securing their systems over the past decades.
Regular Updates + App Store = Safe Mac
Apple launched the App Store - first for the iPhone and iPad - but later for the Mac. It's a huge improvement in providing a safe distribution mechanism for great applications. This coupled with the built-in, regularly updated Mac OS X core is a very safe system for most users.
Regular Updates + Windows Defender = Safe PC
Microsoft is moving towards the App Store model as well. However, there is a larger legacy back catalog of software - so it will take longer for this to be viable for users. However, Microsoft does offer its own free anti-virus system called Windows Defender. Regular scans with Defender should be all the average user needs. It's important that you use the latest versions of Windows 7 or 8.
Finally, while the systems are safe in general use, it is important to practice some intelligent usage habits:
- Don't accept CD's or USB drives from strangers (or those who likely follow unsafe practices)
- Don't browse to shady areas of the Internet (you know what that means)
- Don't allow your browser to install ANYTHING without checking with someone who knows which things are safe
- Keep your browser upgraded. Firefox and Chrome now do this automatically.
- Have your computer checked by pros if it starts acting up. If anything escapes the safeguards discussed above - you aren't going to get rid of it easily!