This question already has an answer here:
I want to use the preinstalled vdi files from virtualboxes.org because it saves me time having it already built for me (what's not to like?).
I don't know if they have viruses on them (they very well might).
I am running a Windows 8.1 host, and I want to use a linux distribution from this site (specifically Damn Small Linux) So does running these files in VirtualBox pose a threat to my windows 8 system?