We have a few PC running XP sp3 have some network problem, if the pc has not been use for some times, it will locked, when user unlocked it, all the network loset, IE FF, outlook, thunderbird, all the windows base program lost the network, but I can ping and trace route from command prompt, to fix it, user just log off and log back in everything would be fine, I try to reset the network card, renew the ip, disable and reenable the network card, nothing worked if I dont log off. any idea what may cause this? your help will be very much appreciated.

So if the PC comes out of the 'locked' mode, and you can ping other hosts, then you know that everything up to layer 3 is working just fine.

Here's the question: When the machine is locked, do you mean screen saver, or do you mean standby/hibernate?

With standby or hibernate, many times, the pc will shut off power to the NIC which can interrupt the windows services. Especially if you leave it off for a long time. The authentication token (access token) can become invalid for a number of reasons especially of you are inactive for an extended period, change networks during the same session.... etc. OF course logging out and in would fix it since the token is created at logon time.

The lock mode is screen saver, we set it up on the domain GP so the PC enter the screen saver mode every 30 mins, and require password on resume, also the NIC setting does not allow windows the turn off its power, because user needs to remote login when pc in standby or hibernate