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.