Wireless using Apple Base Station and Windows

Helping a friend connect a Windows XP Media Center 2002 PC with an Apple
AirPort Extreme Base Station that's on the other end of his house. The PC
is upstairs, the Station is downstairs. It was supposedly working fine for
a while but has been intermittent. The PC has a DLink USB adapter on a cord
positioned up on the desk and has a good path through the door and out into
an open area, though it is a ways to the station. I've turned off the
Norton firewall, but still get limited connectivity warnings and though the
station is set to WEP ASCII 13 character encryption, the PC wants to read
it WPA/PSK and will revert to that by itself after I enter the key. A
windows laptop that connects fine has WPA/PSK (I think that's what it says)
showing in the encrpytion type. I think there are some updates the Media
Center Edition doesn't have that may affect the USB performance possibly??
I saw a warning when I was reinstalling about KB822603. Any suggestions
welcome.