Without knowing your Internet connectivity method, try the following troubleshooting articles:
Q314067 - "How to Troubleshoot TCP/IP Connectivity with Windows XP"
Q314095 - "How to Troubleshoot Possible Causes of Internet Connection Problems"
Limited or No Connectivity - Windows XP Home and Professional SP2 Networking Fix.
Trying to unweave, unwind, unravel
And piece together the past and the future,
T. S. Eliot
Last night I install XP Prof onto a new PC I'm building. Everything was working fine until I started downloading XP updates. After downloading many updates and successfully installing I rebooted as it prompted me. When the PC came back up my internet connection did not work. I'm guessing it was one of the security updates that caused the connection to quit. I did install SP2 but after my internet connection had already quit working. I don't know if this has anything to do with it but it seems that under my network connections I have an internet gateway showing that I don't think is supposed to be there. It seems as though my PC is trying to connect to the internet through another PC. Any help would be appreciated.