I have performed some experiments with wireless network adapter on Windows XP SP3 having Norton Internet Security installed. I can confirm that after installation of WinpkFilter NDIS IM driver wireless connectivity is lost. But it is restored after reboot. Sometimes network stack can’t be rebound dynamically and after installation of NDIS IM driver you may have to reboot. This probably the case your users have met with WinpkFilter (both reported cases had Symantec software installed).