Can't set up dial-up connection

I can't set up a dial-up connection on my sister's hp laptop.
The modem is a Motorola SM 56 Fax and speaker phone modem. I get a message when I try to connect with the AT&T Dial software and when I manually set up the connection " error 633 modem already in use or not configured properly." I am not a computer wiz..but it says the port is COM 3. and when I looked at the log when it failed to connect, it said that "opening modem failed with error 00000003." She has Vista on the laptop. My phone line is ok and it doesn't have call waiting or anything on it..and I connect my other computer to it. I use the line only for my computer. I tried it on a line for my phone and it doesn't work on that either. I set up a connection on another computer running Vista and I had no problem.
I am going nuts over this. She died and my other sister gave me this machine.
Thank you.

Barbara