avast! antivirus Home Edition is available free of charge for non-commercial home use. There is a commercial license available from $39 to $10 per workstation per year. Not for profit organizations can get it at 50% off.
From www.avast.com: “avast! antivirus v4.7 represents a multi-award winning solution to virus protection, with anti-spyware and anti-rootkit software built in for added security. It is available free for non-commercial, home use and with its fast, automatic updates avast! antivirus is already being used by more than 50 million users worldwide to provide continuous protection against all forms of malicious software (malware).”
It is very good and I recommend it.