TCP/IP – loopback failure

I have a network of Win2K, NT, and 95 workstations logging on to a NT4 server. All have internet/e-mail access through an IDSL router using static IP’s. All work fine except one Win2K notebook – all TCP/IP functions work fine (ping name, ping IP address, ping gateway, etc.) except for ping loopback IP (request timed out). I’ve tried 2 different network cards, completely removed TCP/IP (including regedit according Microsoft doc #Q151237), and re-installed TCP/IP – same problem. Can’t access internet or send/receive e-mail. Network logon/connectivity is fine using Netbui.