Windows XP Can you unknowingly be a server?

I recently moved into a new place; it's a roommate situation, and our wifi is in my roommate's name. But my roommate has been threatening to change things so that only he has access to the wifi, and then I'll have to make some other arrangements if I want internet. This is because he's convinced that people are somehow remotely accessing the files on his computer. I asked him why he thinks that, and he says it's because whenever he opens up the command prompt app, it says "host." He's very troubled by this because he doesn't want to be a host. He said that he keeps changing it, but people are somehow remotely changing it back.

This all sounds extremely implausible to me, but I'm a Mac person and I don't know enough about Windows to refute any of this. At some point, I'd like to sit down at his computer and have him show me exactly what he's talking about, but first, I thought someone could at least tell me the meaning of "host" in the command prompt, as I highly doubt there's anything sinister behind it.

One more thing: I'm assuming that in Windows, there's an option to turn on a firewall, right? I asked him if he has a firewall turned on, and he says that he turns it on, but again, these mysterious people are somehow remotely turning it back off. I think he might be confused about what a firewall is.

I'm going to try to get more info about this, but meanwhile, I'd really appreciate it if anyone has any ideas on anything I can say or do to assuage his fears.

He told me he has two systems, one running XP and the other running 8.1.