I've always liked Grisoft's free AVG Anti-Virus program. It's great for home users who need bare-bones virus protection. For commercial users, Grisoft also makes AVG Anti-Virus Professional, which includes spyware and spam protection, a software firewall, and more. For those who already have antivirus software but need antispyware software, Grisoft also offers AVG Anti-Spyware.
In these two galleries, Erik Eckel walks you through the installation and configuration of both AVG Anti-Virus 7.5 and AVG Anti-Spyware 7.5: