Mwenza, before you write off the free computer anti-virus tools, you have to consider all the dynamics of this virus thing.
There are strong sentiments regarding the link between the origin of viruses and the makers of antivirus software. Some argue that there must exist some loose correlation between these two.
Else, the makers of free anti-virus tools need not sale you the direct commodity to churn profits; they could benefit from selling advertising space on their websites based on the "hits" it receives.
Furthermore, considering how a good number of "techies" work, It wouldn't be a surprise for some of them to develop tools without harbouring any commercial motives. I am in the technical field and I can name a good number of free products that are popular and reliable compared to their non-free counterparts.
That said, I personally cycle through the free anti-virus by gauging their effectiveness (this might change with time). As a personal preference, I currently recommend Avira or Avast, especially if you are into torrents!
A trick that works like charm for me is to run whichever anti-virus software alongside Microsoft Security Essentials program (Free also albeit for Genuine Microsoft Operating System , as Microsoft has to fight to protect it's reputation!)
The golden rule is: whatever you use, ensure that you UPDATE IT REGULARLY.
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