This is from my experience and I don't know your full setup but here goes.
I have a 1.6Ghz machine and on a wireless ISP. I was having a lot of trouble getting the connection to work. Finally, after two years of slow performance, then hdd crashing, I had new hdds put in and replaced the OS to 98SE. Then after that was two weeks of tweaking, using fixes, etc, and this is what I did.
If you are NOT using modems check the device mgr for "TCP/IP Adatper" in with the Network Adapters. If there is one there go into properties and disable it. I found my mobo just will not leave it out iwth either 3COM NIC (by the way mine worked fine before hdd crashed) or DataLink NIC. Once disabled the connection works like a charm.
I hope this helps. For other options read "XPH back to 98SE, now ethernet card won't work" in the forums.
Good luck.