Belkin desktop wireless card problem

I am having trouble getting my desktop card to work. I have a Belkin
pre-N router connected to cable. I have a desktop running XP with a
Belkin wireless card on the 1st floor of the house which works fine. I
recently bought another Belkin card for a desktop running W2K on the
2nd floor. Installed s/w, card. Saw good connection. This worked for a
few minutes then stopped. Showed no connection. The icon showed red.
I thought that this was probably because of the distance. So, moved the
desktop closer to the wireless router. On bootup, the connection seems
fine, then goes off. But this is not consistent either. On repeated
reboots of the PC, the signal is still off.
Called Belkin support. They suggested changing the channel on the
router config screen. Did this. Same problem. They suggested I take the
wireless card back and get a replacement.
I then tried with the card on the 1st floor. Same problem. So its not
the card or the distance from the router.
Any ideas?
Thanks
Mark Sabatini