If you're using Windows, Security Essentials plus common sense is fine. Security essentials is even built into Windows 8 and beyond (as Windows Defender). Other security/antivirus programs offer different realtime shields that may be helpful, but those also incur performance costs. If you're using a bit of common sense, these realtime shields are largely useless, so you're taking a performance hit for very little benefit. As far as virus definitions go, there isn't much of a functional difference between different antivirus programs. In some cases, paid programs even have problems with false positives (http://arstechnica.com/business/2010/04/broken-mcafee-dat-update-cripples-windows-workstations/) which in the extreme cases will even break your operating system. Security Essentials is the best choice in my opinion because it provides a good level of security without getting in the way of normal operation. If you don't trust yourself, or prefer the comfort that the realtime shields of another program offer and you're fine with the performance hit, those are fine too.