Can't stop computer from calling my ISP!

[Robert M]

I have a mini-network of two computers running XP (or
should have). One of the computers is connected to
internet through a ISDN connection. It is configured not
to make any automatic dial-up to the ISP, and that works
fine until I connect the two computers (plug in the cable
between them). As soon as the connection between the two
computers is established it starts dialing the ISP, and I
can't figure out why!
Any clues anybody?

Yours R.