|I also agree with T-R-A.|
No AV or firewall software is guaranteed to be 100% effective. But it is very important to have these security elements installed to prevent your machine from being compromised or at least reduce your exposure to such risks. Overall, when these security tool are combined with good security habits, you are guaranteed the minimum exposure.
Another thing to realize is: Installing a reputable security software isn't where it all ends, you need to ensure you are up-to-date with definition releases, as well as software patches applicable to your machine's OS & other apps you have installed to help fix vulnerabilities. Most software come with the option to obtain or install these types of updates automatically. Some prefer automatic updates & many others would rather go with manual install.
There are many top-notch AV vendors out there, some free, but most are not. Among the pay versions, NOD32 & Kaspersky have consistently been in the lead. As far as the free alternatives, you can't go wrong with Avast, AVG has been consistent too. You might also consider Avira or even M$' Security Essentials Beta, which is better than I anticipated being from Redmond.
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