I set up a LAN with two computers both running XP Home. For some reason that I cannot fathom when you log onto the guest computer it sometimes logs onto the internet. A tad annoying to say the least as the host computer is upstairs and we don't know when it does it.
I had this problem before and it turned out that when xp was sharing an internet connection across a network, it seemed to treat is like a router that was permanently connected. If for example Norton autoliveupdate was on, it wouldnt hesitate to dial up the internet.