Hello Anna,
As far as free anti virus programs go, there are quite a few of them out there, some better than others. I personally use the free version of Avast. I used to use AVG for a while, but it has become rather bloated over the past few years and has a much larger "footprint" that Avast, ( that just means that it uses more resources than it really needs to and as a result will slow down your computer a bit, just my opinion)
You can download Avast free from their website, but just watch out as pretty much all the free anti virus software companies will offer you their free version, but they will continually try to get you to upgrade to their paid version. They will usually offer a "Free Trial" that is good for 30 days or so, and then it will expire and you will need to purchase the product if you wish to continue using it.
Don't fall for it, the free versions work just fine for the average user and you do not need to get the paid version with all the additional bells and whistles.
One thing to be aware of with Avast, they require you to register your free version. You just need to click the Registration button in the main Avast screen, and give them your name and an email address, that's about it. You will then be good to go for one year, and when the end of the year is almost up, they will send you warnings that your registration is about to expire. You just need to go back into the registration screen and re-register again with your name and email address.
As I said in the beginning, there are lots of free anti virus programs out there, I've tried quite a few of them, but I keep coming back to Avast, I have yet to get any type of virus while running Avast, I can't say the same of most of the other anti virus programs I've tried over the years.
Also, be wary of what malware program you downloaded as there are some out there that advertise themselves as malware programs, and they end up giving you false information telling you that your computer is infected with malware or viruses, and that you need to pay them to download a program that will help to clean up your computer. There are quite a few of these types of programs out there.
I only use 2 programs to protect my computer from malware and spyware, they are Malwarebytes, free version, and Spybot, also free version. If you computer is running slow and giving you problems, I would recommend downloading Spybot and Malwarebytes free and run them both on your computer to make sure you haven't got a bad malware program on your computer already
Hope this helps and answers your questions, Good Luck!
Mike