Lose Internet connection after some time

When I start my PC I could normally connect to the internet. However after 30 min or so, I am unable to browse using IE or Firefox. But Im still connected on my yahoo. If I refresh my LAN connection (disable and then enable the network adapter), im unable to connect to yahoo either.
I tried renewing my IP (automatically configured) DHCP), flushing my DNS, manually setting DNS, spyware/virus scanning (spysweeper), using winsockfx but im still unable to browse. The weird thing is that I can ping websites (www.google.com or using their IP equivalent). Im also unable to telnet www.google.com 80…
The only work around is restarting the system again.
Any help would be greatly appreciated.

Platform: Windows XP SP2 (currently updated on MS patches)