Clamwin - http://www.clamwin.com/
Why not just add antimalware instead of another antivirus then run online scanner?
1. Online scanner
-- Housecall: http://www.trendsecure.com/portal/en-US/tools/security_tools/housecall
-- ESET: http://www.eset.com/onlinescan/
2. Malware scanner:
-- SUPERAntispyware: http://www.superantispyware.com
-- A-squared Free: http://www.emsisoft.com
-- Malywarebytes: http://www.malwarebytes.org/mbam.php
It has been stated for numerous years, it is good to have a 2nd or 3rd Antivirus Engine to detect viruses your "main" program may have not detected. But, of course you can NOT have both AV applcations running at the same time. Does any SW company still make an application that does not installed and automatically run in the back ground as your "virus shield". Need one where it just installs but scanning is manual operation.