So, no matter what web page you open first, if you try to go back to it after accepting the agreement you get pushed right back to the portal page again.
I think I've seen this before. The controller is doing a DNS redirect for google.com to resolve to the controller's virtual IP address. This is how the web auth screen appears. After you login, for some reason your laptop is caching that lookup, either in the browser or in the OS DNS cache. You can try to use the command
ipconfig /flushdns to clear the local DNS cache in XP
ipconfig/displaydns to see the local cache in XP
I'm not sure how to see what is in your browser DNS cache.