I ran into a problem yesterday with a VPN client not working in XP. After a Usenet search, someone suggested that removing the QoS client (which does nothing useful anyway) fixes the problem about 80% of the time. In my case, that worked.
If something networking-related isn’t working right in WinXP for you, try removing the QoS client and see what happens. At the very least, you’ll speed up networking slightly, and at best, you’ll fix the problem.