You need anti-virus, anti-malware and a firewall.
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Many companies call the combined three their "internet security suite" but they are technically different pieces just bundled together.
In fact, Windows 7 includes a firewall by default but if you purchase IS software they usually turn off W7's firewall and use their own.
Another thing that could be important: The purpose of anti-malware is to keep viruses from ever getting to your machine and to keep malevolent scripts from running. So for the main part of anti-malware to work it must be incorporated with your browser. If your browser of choice is not supported then you are not as protected. For instance if you use Opera and there is no add-in for Opera then you are less protected when using that browser or if you use the 64-bit versions of IE or Firefox but you buy a product like Norton IS which is still only 32-bit then your 64-bit browsers are left unprotected.