Depending on who provides your internet service, you might be able to get a free antivirus through them. I get free Norton through Comcast. Edit: I didn't read the whole thread so I didn't know @86WARD already mentioned that. But yea it's free. And yea I didn't know it was offered until my Grandpa told me about it when I mentioned buying it.
They do it pretty regularly. For instance I just updated mine literally 2 minutes ago. If for some reason you aren't updated, the thing will notify you and put an annoying orange thing on the icon to let you know you are vulnerable.