Most people have a love/hate relationship with Antivirus product. The most common products most people use are typically slow, loaded with useless features and generally provide a bad overall experience. In some cases, simply replacing this "known" products with the never products may actually noticeably speed up your computer.
Since inception, I have been using a FREE antivirus from Microsoft called Microsoft Security Essentials. If you are one of the couragous souls that upgraded to Windows 8 Pro (like me) then it is alreasy built into the OS with its original name Windows Defender.