virus free, antivirus download

I know next to nothing about computers.  I taught myself how to type and search the internet, but if I had to answer any technical questions, I would probably have a really dumb look on my face. So when my computer stopped working this past week I called the expert in, my dad. My dad is always coming to my rescue when it comes to my computer. 
The first time I used my computer, I thought I had picked a good antivirus download, but my computer was not virus free for a month. When I called my dad, he couldn’t stop laughing when I tried to explain what the problem was. I told him I clicked on the antivirus download and then my computer went crazy. As my dad started to ask question, I was almost in tears as I tried to answer them. That was when he decided to just come over and fix my computer himself. He told me, no worries, it would be virus free in no time. He was at my house for hours trying to undo what I did in a matter of minutes. Now I just call him and have him come over anytime I want to do something new to my computer!