I posted about this topic a Few weeks back but never really got to the bottom of this. I eventually managed to cure it with Go Back but now it's back! I get an unwanted dial-up every time I boot, and if I manually disconnect, it tries to connect again in seconds.
I've done a Web search for this and it seems to be a problem that irks a lot of people. There doesn't seem to be an easy solution and the advice is always check for spy ware, check for programmes looking for updates and do a virus scan etc.