On my opinion the best antivirus program existent at this moment. Why?
Simply, due to its advantages: no renewal time limit (you may update virus definitions till you live on this earth or at least till Symantec's bankruptcy), very low RAM memory consumption(18MB), email protection, very quick scan, scheduling scans, intuitive interface. I must mention that the program is just for client and desktop computer. (It will not work on servers).
OS required: all Windows family for desktop and client PC
File size : 20 MB
Install note: download the archive, extract the file" nav ce 8.1 client installer" in a folder and run it.