Showtime Drop- Possible Modems conflict?

Having purchased a Conextant Generic wireless -G Router. I have since set up various PC's to connect to my Main PC using the wireless LAN feature. However...I am unable to get the WAN side into operation I have set up the system according to good advice on the web http://www.cazart.plus.com/conexant/
but I am only able to get the connection to show the SHOWTIME message for a brief second before it drops. Does anyone know if there is a conflict between using an existing broadband modem and installing a new router/modem even if the existing modem (Speedtouch on PIPEX ISP) is unplugged (USB) and disabled under network connections ?
Note PC runs Norton IS 2005 and was disabled for testing. Uninstalling the existing modem driver is for me a dangerous scenario and would want to know what informed peoples advice would be on the probability of conflict In any case I would like the option of swapping between modems if needs be
Any pointers or web pages would be greatly appreciated.

Cheers
Slowie