Problems with secure wireless network

We are using Acer laptops with wireless cards that connect to our network and in a domain.

Connectivity with no security is fine. But if you enable WEP and try to authenticate to access the network, it fails to get an ip address from DHCP server and fails to authenticate with AD userid to to domain and get no connection. We use a Cisco ACS Radius server. The laptops have Windows XP SP2 using Acer IPN2220 wireless lan cards.

Any ideas?