My client has a Dell laptop with inbuilt Intel wireless NIC and onboard Ethernet NIC successfully connecting to a Minitar wireless hub/access point. The AP is connected to an Alcatel Speedtouch Home ADSL modem.
When connected via Ethernet he can access both local devices and browse the Internet. When connected wirelessly he can communicate with local devices on the LAN but cannot connect to the Internet. The laptop is running WinXP SP2 with McAfee Security Center with Firewall.
Another laptop on the same wireless LAN can connect to the Internet about 10% of the time (albeit very slowly) but times out the other 90%.