I've addressed this question to Windows groups, but have had no helpful responses. Since the AP is an AirPort, maybe the problem lies within it?
The "network" consists of a snow AirPort connected to the DSL " modem ". Mac PowerBook G3/400 (Mac OS X 10.3.4) and Dell Inspiron 600M (Win XP Pro) wirelessly connected to the AirPort.
The PowerBook has no trouble at all accessing the 'net, with or without WEP security. The Win XP laptop will connect to the wireless network as long as I don't use WEP security. If I use WEP, while the WLAN card always sees the signal strength as strong and the speed as relatively fast (11 Mbps), the status shows "little or no connectivity", and network apps (ie, Explorer, etc.) can't connect to the 'net. The PowerBook has no trouble with WEP.
The AirPort is in DHCP / NAT mode and has mac address filtering disabled. Attempt to renew DHCP lease in the Win laptop results in a timeout error (can't communicate with the AirPort).