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Microsoft Security Essentials (MSE) and the Windows Firewall are all that you need to protect you. I have been using this methodology for many years and have had no problems.
MSE is great because it has a high malware detection rate as well as not loading down your computer's resources with unnecessary "bloatware" as is common in many software solutions.
Also please make sure that you download any Windows updates that are released for your Operating System (OS W7) and also to do a full weekly MSE anti-virus scan.
Windows updates are released monthly on the second Tuesday.
On my computer, I also have Malwarebytes (MBAM) downloaded in case something was to slip past MSE. You can run MBAM on an ad-hoc basis and an important factor is that it plays nicely with MSE.
You can download the free version from here by clicking the blue button at the bottom of this page
Finally, what you can do to add another level of protection to your Internet surfing, is to make sure that you are using a secure Web Browser such as Internet Explorer and also to set your computer to use a secure free DNS server which filters websites for any links which can lead to malware. I recommend the Comodo secure DNS server which you can read about here: