TCP/IP Stack Woes

I am unable to use our office ISDN line across our network for mail or internet (IE5.5) I have a separate dialup modem and all works perfectly, albeit, slower. I deleted the TCP/IP stack and shut off networking, and then reinstalled – no change.

I can ping, telnet, and ftp separately, but as soon as I use IE or OE, no luck. Any Ideas?