Win 7 or Linux. Really, XP is the most targeted OS for malware. The best prevention is to not use it.
The second best solution is to never connect to the internet or a network, and never share files with a computer that does.
If you still need to get something, PC Mag and Cnet both do frequent reviews and post recommendations:http://securitywatch.pcmag.com/none/306933-choose-the-best-antivirus-in-2013