Well if your using just a normal Anti Virus like a free one, then you can install spybot search and destroy http://www.download.com/Spybot-Search-Destroy/3000-8022_4-10122137.html
and use the tea timer thing which helps stop spyware etc getting into your system, and use the browser immunization as well..
And do you want free anti virus or paid? cos i quiet like norton, the new 2009 version :)
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Chicken