This is a controversial topic. I suggest you visit this web site to get a sense of which AV software does the best job, depending on what kind of user you are: http://www.av-comparatives.org/ This is a non-profit that has been around for a long time and I trust their results. Their latest report: https://www.av-comparatives.org/comparison/
When you interpret these results, keep in mind the difference between a product that detects 95% and another 99% of infections, is actually 5 times more likely to allow an infection.
I am speaking from a great deal of experience. I support 130 client computers. I have been doing this for 17 years. My clients NEVER have infections.
The only products I will use or recommend are ANTIVIRUS-ONLY products. Any product that is called complete protection, Internet security or deems itself to be a comprehensive security product is prohibited on my client computers.
I have good reason for this. Over the years and hundreds of PCs, I’ve learned a lesson well. That it is very difficult to diagnose and fix the problem(s) when components of an Internet Security product are actively working to prevent that diagnosis. The software sees the technician (me in this case) as a threat. I discovered that if I uninstalled that software, I am able to much more likely and quickly discover an fix the problem. Antivirus ONLY products do not behave like this.
The other components in these IS products (other than the AV itself) cause more problems than any benefit they bring and mostly work by shutting down the Windows component that does a very good job of that particular task anyway.
It is the Antivirus software that you need. It is actually all that you need with Windows 7.
Most of the big name suppliers much prefer to sell their all-in products because they can sell them for a higher price. Some do not offer an antivirus-only product. Some push their all in ones and still sell the AV only product, but you may have to look for them. There is at least one and it is either the best or close to it.
Beware of products that try to actively engage you in the security process by asking your permission to run a program named xd43_45d*.exe, or the like. Also look out for products that report a lot of false positives, misleading you into believing you have an infection when in fact, you do not.
Do not use any product that claims to make your computer run better. Virtually all of them are either bogus, themselves an infection, or put your computer at risk. Remove everything that claims to be there to prevent infections or claims to make your computer run better, with the singular exception of just one antivirus product.
Note well. You should never have more than one protection product installed. Always ensure that you completely remove any and all protection software packages before installing a new one. That includes software that may have come pre-installed on your computer when new. Some products will conflict with one another, causing yet more problems.
You cannot fully remove most any AV software by simply using the standard Windows 7 uninstall tool. Most of the majors also publish a “removal tool” that gets the rest of it. If you do not completely remove it, it will cause very hard to understand and diagnose problems.
The “Security” software installed on your computer may be causing problems that can not be diagnosed or fixed. Try removing it temporarily.