It's possible that it's some sort of worm... however seems rather an odd worm to create, then again worm creators aren't exactly normal.

Is your computer having problems online? i.e. do you type in some web addresses, such as Amazon.com and get redirected to a porn site or some other less than reputable sites?

Any other oddities i.e. downloads being extremely slow (besides from that file), websites taking ages to load etc.?