Standalone Virus Scanners

I use Trend Micro's free HouseCall on demand online scanner as a 'second opinion' sweep of any system I suspect may be infected. Every PC I've encountered with a virus had some form of antivirus software installed, which give an all clear after a scan.  Unlike some online scanners, HouseCall will attempt to remove any malware it finds.  See: http://housecall.trendmicro.com
Other alternatives are the Microsoft Safety Scanner and ESET Online Scanner