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There is no such 100% effective antivirus program because everyday 50000 of viruses come out and with this fact every AV is just statistically effective so you should be exercising self caution that means your best side by buddy to avast should be your brian. ;D
My numbers are bit low…the number is now 100 000 + viruses everyday
The malware numbers game: how many viruses are out there?
http://www.zdnet.com/blog/bott/the-malware-numbers-game-how-many-viruses-are-out-there/4783
http://www.mcafee.com/in/about/news/2012/q3/20120904-01.aspx