A lot of malware these days is able to detect when it is running virtualized under VMWare, VirtualPC, WINE, or even in a sandbox such as Anubis or CWSandBox.
This essentially means that malware will often "hold back" or not function maliciously when running in a virtual environment in order to thwart analysis of its true intentions.
My thought is then, why not make your PC appear as if it is virtualized? Does anyone know how I might be able to go about this?