avast! 4 Home Edition is a complete ICSA certified and Secure Computing Magazine awarded antivirus software for home noncommercial use. Avast includes the following components: On Demand Scanner--with skinnable simple interface--just select what do you want to scan in which way and press the Play button; On Access Scanner--special providers to protect the most of available e-mail clients; Instant Messaging--ICQ, Miranda; Network traffic--intrusion detection--lightweight firewall; P2P protection for Kazaa, BitTorrent; Web shield--monitors and filter all HTTP traffic; NNTP Scanner--scans all Usenet Newsgroup traffic and all operations with files on PC; Boot Time Scanner--scans disks in the same way and in the same time as Windows CHKDSK does.

Version 4.7.1001 is fully compatible with Outlook 2007, it contains new unpackers and is available in 3 new languages.

Note:
You'll need to register this program once a year to use it free, and the interface is a bit tacky, but it quickly scans your computer for viruses without hogging too much memory.