Well, the way I see it, the machine would have to have a level of self-sustainment, such as being located in a place simply really annoying to go to. Then it would need a large number of resources for spare parts, and machines to repair/recycle parts, a way to gather new materials in case of extended use, and a way to update itself, such as finding flaws, and designing a way to fix it; in short, AI-induced mechanical evolution.

Second. It's a post apocalyptic scenario, so let's just say this virus made this thing hack some launch codes and nuked the world. On accident, of course.

By the time survivors are able to interact with the nuclear world they live on, they probably won't know how to operate the machine. "ctrl+alt+del" is old world magic they don't understand.