Multiple users have seen this error on our network. They will be working along on the internet or in office etc, then all of the sudden they get a Dialup Connection window that pops up and asks them to connect to the network. This doesn't happen on all the machines in a network segment, only a handful so I've eliminated problems with switches. I have noticed that when this happens to a user, they lose their network adapter for a moment. Once the network adapter comes back to life then everything works fine. Could there be a virus out there that randomly disables your network adapter? Since there is a wide scope of machine types out there that are having this problem I can't think of anything hardware related that would cause this.
Maybe there was an update from MS that automatically attempts to "Repair" the network adapter when it detects slowness or an error?