This was a brand new PC running the latest version of Windows and fully patched. There were two well known AV programs installed not to mention Microsoft Security Essentials. The reason she was infected is simple: "click this box to save the world". No amount of security in the world will protect people from their own infallible minds. We can try to protect people from all sorts of threats in the world; virus, terrorists, crime, even taxes but in the end we are all victims of our own gullible minds. Let's face it, if bad people can design a web page to look legit, they can just as easily make an application look identical to any number of seemingly safe applications. One errant click and "it's goodbye Seattle" --Steve Martin.
Microsoft Security Scanner is a frequently updated tool (it is only valid for 10 days) that can be downloaded and copied to an infected system either in safe mode or from a secondary boot method. This tool should find and remove most malware or virus.