Going for free antivirus software instead of a commercial program saves you money - however, you have to make sure that you really get the level of protection you need. If you want to achieve comprehensive protection comparable to that of paid antiviruses, here are the things you need to take into account when choosing your free antivirus.
At a Glance:
1. To fully protect your PC, you need antivirus, anti-spyware and firewall protection.
2. Make sure you get these all-important features.
3. Additional important features to consider.
4. Check the detection rates!
5. Get adequate update frequency / size.
6. Hardware / Software considerations.
7. Is some free support available?
8. Is it easy to use?
9. Don't get malware instead of antivirus!
1. Make sure your PC is fully protected
If you decide to protect your computer from viruses for free, the main thing to keep in mind is that one free product is normally NOT enough to achieve full PC protection.
To fully protect your computer, you need at least the following three components:
antivirus - protection against computer viruses, anti-spyware - protection against malicious software, firewall - protection from direct Internet attacks.
Paid anti virus programs often include all of the above components; however, free programs normally contain just one or two. So, free anti virus software will often do just that – protect your PC from viruses. To protect your computer from Internet attacks and to take care of spyware and other malicious applications, you will need to download and install additional free software.
So, always check which of the these components - anti virus, anti spyware, firewall - your chosen software contains; then download and install the missing ones.
Always make sure that the applications are compatible with each other (i.e. can work together) before installing the software. For example, an anti virus program from one maker may not be compatible with a firewall from another maker. This information is normally available on the makers' sites.
2. Check for these all-important features
Make sure that your antivirus has the following important features that almost all antiviruses have:
Real-time protection - automatic protection provided by the part of the antivirus that is always on and monitors your computer for suspicious activity. On-demand scanner - in contrast to the real-time protection, on-demand scans are initiated by the user with the purpose of finding inactive viruses or other malicious programs. Auto updater - with new viruses being released nearly every hour, it is extremely important to choose an antivirus program that auto updates itself regularly (see Adequate update frequency). Heuristics - heuristic analysis provides extra security by detecting and neutralizing previously unknown viruses.
3. Additional features to consider
Ask yourself what additional features you require in your free anti -virus software. Some of the main additional features to consider normally are:
Email protection - some free antivirus programs are not capable of e-mail scanning. As many security threats (viruses, phishing scams etc.) arrive via e-mail, it is very important that your anti -virus has at least a basic e-mail scanning capability. Instant messenger or P2P protection - if you use Instant messengers ("chat" programs like Yahoo, MSN, ICQ etc.) or P2P (peer-to-peer programs like eDonkey) applications, you may want to check whether this feature is available. Scan scheduler - allows to schedule anti virus scans at set times and at set time intervals. Reporting tools - some anti viruses not only log in all events, but can notify you of their actions by e-mai or other means.
4. Your system - hardware/software considerations
What operating system are you using? Your choice of antivirus software will often depend on that. For example, some antiviruses will only run on Windows XP, and some are not ready for Windows Vista yet. Are you running an older / entry level hardware? If so, you should find out the recommended hardware configuration for your chosen antivirus. You may want to opt for antivirus with a small footprint and only the very necessary features.
5. Detection rates
Obviously, detection rate is one of the main factors to consider when choosing antivirus software. Always make sure that your antivirus of choice is able to detect as many viruses as possible, and achieves good results in the following tests:
6. Adequate update frequency / size
Basically, “the more the better” principle applies here: the more frequently your antivirus updates itself, the more reliable your protection is. However, if your Internet connection is slow, frequent antivirus updates – and the large size of update files - can be a problem. In this case it is a good idea to choose an antivirus program with smaller updates.
7. Support
Look for free antivirus software that comes at least with some form of free support. Usually, either a dedicated support forum or email support is the most you can get for free, as phone support is normally reserved for paying clients.
8. Ease of use
Your antivirus software is going to be used every day – so it should be easy to use. It is a good idea to check screenshots, online walk-through demos and reviews of the program before installing it. Also, it is good to know how intrusive it is – some programs will display “nag screens” prompting you to buy the commercial version more often than others.
9. Is it malware itself? We live in a crazy world, and sometimes programs that claim to be anti spyware or antivirus can themselves be malicious/rouge software. Yes, it is possible! The web sites like Spyware Warrior keep lists of such software. Be sure to check your chosen antivirus against these lists before installing it.
How to choose free antivirus software 
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