Another one you might want to try is Comodo Firewall Pro. It is a free firewall that includes a system they call Defense + - not sure what that type of function is called, but it monitors the behavior of all applications as they run on the computer, and alerts you to suspicious behavior. The software does trigger a lot of warnings, so I don't usually run it unless I am looking for a virus or other malware.

On a related note, my father recently got a particularly aggressive virus (this one is generally referred to simply as the Antivirus 2009 virus, a form of scareware) and one of the things it did was change the links in Google SERPs. However, another thing it did was interfere in the download of anti-malware software. As this might be an indication of an infection, you might want to download the software on a clean computer, and burn it to CD, then install it on the possibly infected system from the CD.

As far as Avast, one thing that is common with viruses today is that they will neuter common antivirus applications. They may delete or corrupt the definitions file, or they may trick the antivirus software into downloding updates from their own server instead of from the antivirus company. This renders your antivirus software useless. The remedy for this is to fully uninstall your antivirus software, and reinstall it from a CD created on a clean computer. Additionally, you would want to burn the latest virus definitions file onto the same CD, and force the software to update from there instead of over the web.

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The best way to learn anything, is to question everything.