There are little software programs which interfere with computer operation and they are called viruses. Viruses might cause damages, spoil or delete data on the computer. They extend from the computer to another computer.
They use e-mail programs to extend itself to another computer.
Everybody may receive viruses with e-mail messages or instant messages, images, greeting cards or audio and video files. They can be hidden in programs you might download. . And they even can erase everything from the hard disk of the computer.
So, it is very important to protect the computer from viruses.

The market has a lot of antivirus software. I have had several programs and the worst one I had was Norton anti-virus. I used it for 4 month, it slowed down my system and it did not catch a virus and in the end, my system has been spoiled.
Then I tried BitDefender 10, but it slowed my PC. This program detected viruses, but it could not delete them. So, I quieted it because I had troubles using it. I did not understand how it works.

I think that Kaspersky is the best anti-virus program on the market today according to several tests carried out by many different specialists. Kaspersky is not free, but it is cheap in comparison to other anti-virus programs. It also offers a complete pack, which includes firewall, antivirus and spy ware protection.

Among free anti-virus programs one of the best is AVG. It works excellent.
But I think it is useless to compare Kaspersky and AVG, because these two programs are focusing on different field of dangerous. Kaspersky is the perfect anti-virus soft, while AVG concern more about spy ware. So, I think that the best solution to install them both.
And the basic rule, regardless of which antivirus you use is to update virus definitions regularly and send suspicious files to anti-virus companies for further analysis.