ISDN problem with XP

HI guys-wonder if anyone can help.
I am a tech support guy and have a problem with a customer using BT Speedway ISDN with windows XP.
He can connect fine (dual channel) through Network Connections and then browse the web and download emails.
But when he connects through IE or OE it says he is connected at the normal speed but an authentication window pops up saying verifying username and password. So he is actually connected but this small auth window remains on the screen and if he closes it, the connection drops.
The guy had no problems on his Win2K machine either and the ISP I work for fully supports dual channel connection.
Any ideas anyone?
Bearing in mind I am a tech and done all the obvious....no wanting to sound full of it but to avoid replies like 'have you tried rebooting!!'
Cheers guys ;-)

Is this an always-on connection? Does he need to login to access your service? Is the authentication window that he sees for your service? These may seem like stupid questions, but I have cable and when I switched from dialup, sometimes XP would try to sign me on the dialup service when I would start up IE even though I was connected to the Internet through the cable modem (This problem didn't happen in 98 on my other computer). By deleting the dialup connection, I've had no problems since.

All this guy's ever had is ISDN and so has no dialup's. I too had this problem with my cable but this is not the case in this instance unfortunately...
Any other ideas?