XP Slow on network after installing T1

[Cams]

I have just replaced an ISDN with a T1 on a network with one XP box and the rest Win98 running a simple peer-to-peer. The T1 is running through a WatchGuard Firewall. I have addressed the IP on each machine and all the 98 machines are running great, but the XP box is very slow in displaying web sites. It seems to find the sites immediately, but then takes a long time to display the page. Also, files requested by the 98 boxes from the XP are fast, but file browsing the other way is very slow too. In another matter that may or may not be related, on all machines it seems to take a much longer time to connect to the email server (external ISP). Once connected though the emails download very fast. Ironically, if I connect to the email server through mail2web.com the response is immediate.

Thanks for any suggestions.