I would guess that issues related to the PC in question are interfering with wireless connectivity. Try turning off firewalls and WEP/WPA. Test the PC at another WiFi hotspot, you may have troubles with the adapter.
I've tried everything I can think of. I'm using Cisco's own utility to configure the adapter, I've added its MAC address to the router's list of accepted devices, I've tried using Windows' networking tools, and it's using the correct network key and type of authorization...
I got my other computer connected to the network with minimal trouble, so I'm kind of confounded by how difficult the other computer is being. I know the adapter works, because I used it with another wireless network just a few weeks ago. Please tell me there's something silly I'm forgetting. Or something not-so-silly that I just haven't thought of yet.
Both computers are using Windows XP, although I may try Win2000 on the one that's not cooperating, just for kicks.
Thanks for any ideas!