You really can protect your PC with free software, but there's a price. Multiple free security programs usually aren't as easy to use as one pricey suite. Your less likely to get decent support (perhaps I should say even less likely). And they lack features like antispam and child protection which you may or may not need. All of the companies that give away security programs sell more feature-rich versions, which come with better support and additional features.
On the other hand, they're free. And they have less overhead than the big suites, so they won't slow down your PC as much.
What do you need? First and foremost, a good firewall (the one that comes with Windows doesn't qualify) and real-time antivirus protection. (The term antivirus is somewhat misleading. In their current versions, these programs protect you from all sorts of malware, not just viruses. See "Is It a Virus?" for more on the terminology.) It's also a good idea to get a second malware fighter--one that you don't run in real-time. It's like going to another doctor for a second opinion--except it doesn't cost anything.
Start with the Comodo firewall. It regular beats out the not-for-free firewalls in independent tests. It's not the easiest firewall in the world--it gets in your face an awful lot with questions that would confuse most novices--but PC World readers should be able to handle it.
Comodo recently added antivirus, making itself the first free suite. But I've yet to see any independent test results on Comodo's antivirus, and therefore don't trust it. So, when you install Comodo, uncheck the Antivirus option and just install the firewall.
That means you need separate antivirus software. Go with Avast. It's free, uninstrusive, and did very well on AV-Comparitive's last round of tests.
I'll give you two choices for extra protection, and there's no real reason not use both of them. They are SuperAntiSpyware and Malwarebytes. Scan your hard drive with one or the other of them every week or so--and if you suspect something.
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