On the free side, Microsoft Security Essentials and Malwarebytes Anti-Malware is about all you need. While some swear by Avast! or NOD32, there is very little reason to go for paid AV protection anymore.
AVG Free edition is a very good overall protection suite and does a good job of protecting your system while remaining fairly unobtrusive.
Also what Internet provider do you use ? they usually also include anti virus - spyware tools for free with your internet service subscription nowadays !
AT&T Yahoo! Mail automatically scans all incoming and outgoing email attachments for known viruses. We use industry-leading Norton AntiVirus software, so you don't have to do a thing. Download the attachment and wait for that reassuring message "No Virus Threat Detected." If a virus is detected, it will be cleaned if possible, and you'll be given further instructions.
I still pay for Norton Internet Security Suite. The latest version has been significantly improved to be less intrusive on the system. I just believe that they are still better at identifying new threats as they come out and supplying updates for them. However, I buy it from a reputable seller off of Ebay for a lot less than they charge.