Do you know that your anti-virus solution is working ? You could try to test your anti-virus program against the EICAR (European Expert Group for IT-Security) test file to see if it reacts the way it should. All you need to do is create the testfile and scan it with your virus scanner. If the scanner detects something it is working as intended, if it does not you should consider changing immediately to a more reliable one.
Antivirus Test
Everyone seems to favor a different antivirus software and its hard to make a choice because of this. A little test from the guys at overclockers.com might help you in your decision. They tested Desktop and Online Antivirus solutions and give recommendations at the end.
