Wep Key setting

Help. Have MN700 Basestation with a 510 wireless USB on
PC downstairs. Initial installation was good and all
worked ok. 3 weeks ago, PC w/USB wireless stops working.
Reinstalled both sides, but cannot get WEP key to be
recognized by basestation. 510 does see basestation since
I have changed SSID. Runing XP on PC and ME on basestation
PC.
Any help or suggestions?
Tom
BTW, DSL modem is set at PPP, not bridge, basestation is
Dynamic on WAN setting. Basestation PC works ok.