First off, do you mean anti-virus or do you actually mean anti-malware? If you mean anti-virus, there are tons of products out there. I personally feel the most paid programs do a better job, but tend to bog down your system. Free programs such as Avast, AVG, Microsoft Security Essentials, etc. tend to be a little better at keeping a bearable user exprerience. I use AVG or Avast on all my computers and I have yet to get a virus that they couldn't handle. That said, I would recommend one of them.
If you truly mean anti-malware, I feel Malwarebytes (paid or not) is the best one out there these days. There is also Ad-Aware, Kasperky has a program out there, Hitman Pro, Bitdefender, and many more. Most antivirus programs state they deal with viruses as well as malware these days so getting a decent anti-virus might help as well. I would check www.download.com and search for "antivirus" or "antimalware" for some good options. Make sure to read the ratings and review before downloading to make sure it fits your needs.
Hope this helps!
Best regards,
Protoman