I haven't had any expierences with free security software, (well actually one, but it turned out to be a virus itself...), but my guess is that is probably no near as effective as programs such as Norton or Webroot that you must pay for. Viruses can be very tricky things, and the security software might not be able to take care of it, you might have to get your hard drive reformatted to be rid of it (or them) permanentely.