Scan using any of the following:

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

>>I thought the purpose of having AV was to prevent things from getting on your computer. Is that a naive assumption?
Depends on how the infection arrives. Depends on how the user respond. Not all type of risk and malware is detected by single scanner (free or paid scanners).