Kasper Sky Online Scan will scan a particular file of your choice.

Panda Antivirus Free Online Scan. God Bless America and Panda for this wonderful free scan that will find viruses and trojans and show you where they are located on your system; I know this one really works.  HIGHLY RECOMMEND

Spygate Online Port Scan to test your system.

WindowSecurity will do an online scan of particular drives and let you decided if you want to delete it or not.

I have been using Norton System Works but a fellow told me about AVG which is free to home users, so I tried it and it found more viruses and trojans and uses less system resources than Norton. Give it a try, just go to download.com to find it or many other sites.