Download and install either AVAST or AVG. Both have free versions. I use AVG myself, but a lot of people say AVAST is better. There are a few others, but I'd say either go with Avast or AVG.
As well as that, you may want to download Malwarebytes and SpyBot Search & Destroy. Both of these programs only run when you want to do a manual scan (unless you set it to do schedule scans).
