Short story plot: Narrator notices that the quality of a friend's BBS or Usenet posts has improved -- just expanded diction and perfect spelling, at first, I think. He finds out his friend has been using some unknown-brand modem (I think it was mentioned as having an unsettling red indicator LED), which seems to be improving his posts automatically. Evidence of self-aware AI mounts, and after some panicked last communication from the owner to the narrator, it appeared to have killed the owner -- I think maybe by triggering an electrical fire -- but that couldn't be proven.
I have some recollection of a device killing it's owner by using a basilisk/Snowcrash-like image to transmit a mind virus that causes the owner to kill himself by sticking his head in a microwave oven with a defeated safety interlock, but I think that element was from some other story. This element is indeed from a different story, "Press Enter ■" IDed in the comment by JAB, below.)
This was probably in an Analog, Asimov's, or F&SF from the '80s, although it could have also been elsewhere and appeared in some collection.