Windows Zero Config - mysteriously switches from WPA to WEP?

Switched wireless router network from WEP to WPA-PSK. Two computers can connect to the network fine, but one is having trouble. Whenever I set up the network as a WPA-PSK and try to connect, Windows Zero Configuration fails to connect and switches the network's settings to WEP. I've updated the wireless card's (Dynex Wireless G for desktop) drivers and am using Windows XP with service pack 2. Any ideas on what's the problem?