Has anyone switched out the motherboard? I did not see that step in the OP message. The slot for the WLAN card could be damaged or underpowered due to a fault on the MOBO. I have seen a case recently where a starved MOBO has funky issues like USB dying and peripherals dying due to the power supply being inadequate.

Since people have had issues in both XP and Vista, I'm inclined to think the culprit is something in the laptop itself.